LeetCode_Solutions

Record my solutions to LeetCode problems

Note: The solutions are not necessarily optimal.

Note: The cost information in each problem is for reference only, since I found that even if it was submitted with exactly the same code at different times, the time efficiency and memory efficiency will vary a lot.

Statistics

Problem ID	Problem Description	My Solution	Difficulty	Problem Type	Accepted Date	Note
0001	Two Sum	Java	Easy	Array, Hash Table	5/23/2023
0002	Add Two Numbers	Java	Medium	Linked List, Math, Recursion	6/15/2023	I made a mistake in this problem which cost me a long time to fix it: Changing the pointer of the next node DOES NOT mean having changed the next pointer of the current node. In detail, we have to use "node.next = newNode"; however, "node = node.next; node = newNode" will not work. The solution 2 is really ingenious and its logic is really easy to understand.
0003	Longest Substring Without Repeating Characters	Java	Medium	String, Hash Table, Sliding Window	5/23/2023
0007	Reverse Integer	Java	Medium	Math	6/15/2023	The idea of reversing a problem is really important. E.g. "rvs <= (Integer.MAX_VALUE - remainder) / 10" is equivalent to "rvs * 10 + remainder <= Integer.MAX_VALUE" (but the latter one will not work).
0008	String to Integer (atoi)	Java	Medium	String	6/15/2023
0009	Palindrome Number	Java	Easy	Math	5/27/2023
0011	Container With Most Water	Java	Medium	Array, Two Pointers, Greedy	6/16/2023	The reason why to move the pointer which points to smaller height is important.
0012	Integer to Roman	Java	Medium	Hash Table, Math, String	4/8/2024	这题切勿用分支来进行判断，而是应该用两个数组/HashMap来实现。
0013	Roman to Integer	Java	Easy	String, Math, Hash Table	5/25/2023
0015	3Sum	Java	Medium	Array, Two Pointers, Sorting	6/15/2023	DO NOT use triple-nested loops (the brute-force attack), otherwise you will encounter runtime error.
0016	3Sum Closest	Java	Medium	Array, Two Pointers, Sorting	4/12/2024	运用排序 + 三指针来解题。
0017	Letter Combinations of a Phone Number	Java	Medium	Hash Table, String, Backtracking	4/12/2024	The key point is using the hashmap data structure and forEach loop of String.
0020	Valid Parentheses	Java	Easy	String, Stack	5/26/2023
0021	Merge Two Sorted Lists	Java	Easy	Linked List, Recursion	6/13/2023	It challenged me for a while on my flight to Seattle. But I had so much fun solving this problem and I learnt a lot. Worth a try!
0022	Generate Parentheses	Java	Medium	String, Dynamic Programming, Backtracking	4/13/2024	这道题大多数题解采用了 backtracking 的方法解题，个人觉得复杂且有点难理解。深受某个评论的启发，得出此解：每多一对括号，只需在之前得到的结果里，尝试在每一个结果的每个位置（字符串开头 - 字符串最后）依次添加"()"，并去重来解决（采用 HashSet 数据结构即可）。
0026	Remove Duplicates from Sorted Array	Java	Easy	Array, Two Pointers	6/12/2023
0027	Remove Element	Java	Easy	Array, Two Pointers	6/12/2023
0028	Find the Index of the First Occurrence in a String	Java	Easy	String, String Matching, Two Pointers	5/27/2023
0033	Search in Rotated Sorted Array	Java	Medium	Array, Binary Search	6/18/2023
0034	Find First and Last Position of Element in Sorted Array	Java	Medium	Array, Binary Search	6/21/2023
0035	Search Insert Position	Java	Easy	Array, Binary Search	5/27/2023
0036	Valid Sudoku	Java	Medium	Array, Hash Table, Matrix	6/21/2023	The solution 2 which utilizes HashSet structure is really subtle.
0038	Count and Say	Java	Medium	String	4/13/2024	再次复习 String 对象的 for each 循环 - 调用 String 对象的 toCharArray() 方法。
0039	Combination Sum	Java	Medium	Array, Backtracking	4/30/2024	这个题目用的是 backtracking，即回溯法。解题思路是将所有的组合都尝试一遍，和为 target 组合就加入到结果里。
0040	Combination Sum II	Java	Medium	Array, Backtracking	4/30/2024	这道题目和 0039 很相似，但是 candidates 数组有重复的数字，且每个数字只能使用一次，加上 candiates 数组的长度比 0039 大，因此如果只是修改 dfs 函数的 left 索引值，会导致超时，这题需要用到剪枝（pruning）的技巧。
0046	Permutations	Java	Medium	Array, Backtracking	4/30/2024	这道题目和 0039 相似，解题思路也是非常类似的。flags 数组其实可以改成 boolean 类型，因为之前的方法用的是数组中 1 的个数来判断是否到达 base case，但是其实没有必要，可以直接通过 path 的结点个数来判断即可，因此这部分其实可以优化一下。
0047	Permutations II	Java	Medium	Array, Backtracking	4/30/2024	这道题目和 0046 基本一致，因为 nums 的大小最大为 8，所以不剪枝也不会超时。剪枝的方法也放到最后作为注释了。
0048	Rotate Image	Java	Medium	Array, Math, Matrix	5/1/2024	这个题目有一定的技巧性，如向右旋转 90° 实质上是先上下翻转，然后沿着对角线翻转。同理，向左旋转 90° 其实是其逆过程，先沿着对角线翻转，然后上下翻转。如果是向右翻转 180°，那就是先上下翻转，然后左右翻转。以此类推。。
0049	Group Anagrams	Java	Medium	Array, Hash Table, String, Sorting	6/22/2023	The main idea of the solution is a little similar to 0036.
0050	Pow(x, n)	Java	Medium	Math, Recursion	5/1/2024	这个题目的解题关键点在于将幂 n 逐步分解为 n/2 -> n/4 -> n/8 -> ...，同时还得注意 n 的正负性和奇偶性。
0053	Maximum Subarray	Java	Medium	Array, Divide and Conquer, Dynamic Programming	6/18/2023
0054	Spiral Matrix	Java	Medium	Array, Matrix, Simulation	5/1/2024	这道题目解题要点在于掌握移动的方向和边界条件。我们可以通过四个变量来记录上下左右边界的位置，从而简便地解题（无需设置一个二维数组记录移动方向）。
0055	Jump Game	Java	Medium	Array, Dynamic Programming, Greedy	6/21/2023	The solution 1 plus is really subtle since it simplifies the condition of the "for" loop.
0056	Merge Intervals	Java	Medium	Array, Sorting	6/22/2023	Similar to 0057. The method to sort the 2d array and the method to convert a list into an array are really subtle!!!
0057	Insert Interval	Java	Medium	Array	6/19/2023	A little bit tricky! The key idea of my solution is to discuss the start point and end point of the new interval separately.
0058	Length of Last Word	Java	Easy	String	5/27/2023
0059	Spiral Matrix II	Java	Medium	Array, Matrix, Simulation	5/1/2024	这道题目是 0054 的变式，是其逆过程，解法是类似的，也是通过上下边界和左右边界来逐步缩小来解题的。
0061	Rotate List	Java	Medium	Linked List, Two Pointers	5/1/2024	这个题目可以通过找规律来解题：每移动一次关注新尾巴的变化，移动第一次，新 tail 为原来倒数第二个结点（正数总结点数量 - 1 的位置），移动第二次，新 tail 为原来倒数第三个结点（正数总结点数量 - 2 的位置），以此类推。。
0062	Unique Paths	Java	Medium	Math, Dynamic Programming, Combinatorics	5/3/2024	这个题目看似用组合数计算比较简便，但是实际上用循环来实现的时候有雷点，如循环控制变量的初始值，以及如何用一个循环来实现等等。推荐用动态规划来实现更简单。
0066	Plus One	Java	Easy	Array, Math	6/8/2023
0067	Add Binary	Java	Easy	String, Math, Bit Manipulation	5/27/2023
0069	Sqrt(x)	Java	Easy	Math, Binary Search	5/28/2023	Similar to 0035.
0070	Climbing Stairs	Java	Easy	Math, Dynamic Programming, Memoization	5/28/2023
0072	Edit Distance	Java	Medium	String, Dynamic Programming	1/28/2024
0074	Search a 2D Matrix	Java	Medium	Array, Binary Search, Matrix	6/18/2023	The solution 2 is really easy to understand and ingenious.
0075	Sort Colors	Java	Medium	Array, Two Pointers, Sorting	6/30/2023	Actually I do not think it is a medium problem but an easy one.
0081	Search in Rotated Sorted Array II	Java	Medium	Array, Binary Search	9/8/2023	The enhanced version of 0033. It is not worth a try since if you would like to use binary search, you would find that the time complexity is the same as the brutal-force method, which is sequential search.
0083	Remove Duplicates from Sorted List	Java	Easy	Linked List	6/14/2023
0088	Merge Sorted Array	Java	Easy	Array, Two Pointers, Sorting	5/29/2023
0090	Subsets II	Java	Medium	Array, Backtracking, Bit Manipulation	6/21/2023	The statement of this problem is unclear since the number of the examples is too small to clarify it.
0094	Binary Tree Inorder Traversal	Java	Easy	Stack, Tree, Depth-First Search, Binary Tree	6/5/2023	Typical knowledge point and frequently be tested in interviews. (DFS in Tree)
0095	Unique Binary Search Trees II	Java	Medium	Dynamic Programming, Backtracking, Tree, Binary Search Tree, Binary Tree	9/26/2024	这道题目运用分治法的思想，将 1 ~ n 问题规模逐渐缩小：分别以 1 ~ n 为 root，当以 i 为 root 时，左子树则为 1 ~ (i - 1)，右子树为 (i + 1) ~ n。
0096	Unique Binary Search Trees	Java	Medium	Math, Dynamic Programming, Tree, Binary Search Tree, Binary Tree	9/30/2024	这道题目如果用普通的递归会导致超时，因此我们需要利用前面计算出的结果来推后面的结果，也就是找出动态规划的推导式，一旦找出推导式了，那么这个问题就迎刃而解了。
0098	Validate Binary Search Tree	Java	Medium	Tree, Depth-First Search, Binary Search Tree, Binary Tree	6/26/2023	The first solution is a little bit tricky about the initial minimum and maximum value, since the value of the node can be Integer.MIN_VALUE or Integer.MAX_VALUE.
0100	Same Tree	Java	Easy	Tree, Depth-First Search, Breadth-First Search, Binary Tree	6/14/2023	An easy but interesting recursive tree-related problem.
0101	Symmetric Tree	Java	Easy	Tree, Depth-First Search, Breadth-First Search, Binary Tree	6/6/2023	Tree-related. Quite interesting.
0104	Maximum Depth of Binary Tree	Java	Easy	Tree, Depth-First Search, Breadth-First Search, Binary Tree	6/9/2023
0108	Convert Sorted Array to Binary Search Tree	Java	Easy	Array, Divide and Conquer, Tree, Binary Search Tree, Binary Tree	7/21/2024	这道题目虽然是 easy 题目，但是解题的思路很重要：需要将待解的问题分为左半部分和右半部分（分治法），知道了用分治法还不够，因为 helper function 的写法很重要。在这道题目里，helper function 返回创建的结点比较简单，而不是在函数创建结点并赋值给 left 和 right 指针（函数值返回为 void）。后者会使得整个解法比较复杂。
0110	Balanced Binary Tree	Java	Easy	Tree, Depth-First Search, Binary Tree	6/6/2023	Tree-related. The solution 2 is really ingenious. The method to get the depth of a tree node is fantastic!
0111	Minimum Depth of Binary Tree	Java	Easy	Tree, Depth-First Search, Breadth-First Search, Binary Tree	7/21/2024	解法一到解法二的优化很重要：当一种情况的写法很复杂时，考虑其相反情况的写法会不会更容易。
0112	Path Sum	Java	Easy	Tree, Depth-First Search, Breadth-First Search, Binary Tree	6/20/2023
0113	Path Sum II	Java	Medium	Backtracking, Tree, Depth-First Search, Binary Tree	6/20/2023	The variant of 0112. One tricky point of this problem is that, once we update the value of a list, the original reference will update at the same time. To be more specific, "result.add(current);", where both result and current are of type List, if we update the value of current, then the value of result will update.
0118	Pascal's Triangle	Java	Easy	Array, Dynamic Programming	5/28/2023
0119	Pascal's Triangle II	Java	Easy	Array, Dynamic Programming	5/28/2023	The variant of 0118.
0121	Best Time to Buy and Sell Stock	Java	Easy	Array, Dynamic Programming	5/28/2023	A little bit tricky!
0125	Valid Palindrome	Java	Easy	Two Pointers, String	5/27/2023
0128	Longest Consecutive Sequence	Java	Medium	Array, Hash Table, Union Find	6/16/2023
0133	Clone Graph	Java	Medium	Hash Table, Depth-First Search, Breadth-First Search, Graph	9/27/2024	这道题目因为图是一个无向图，所以不同结点的邻居可以是相同结点，因此需要在创建 clone 结点的时候记录其值，具体方法为创建一个 HashMap，记录原结点和 clone 结点之间的对应关系。
0136	Single Number	Java	Easy	Array, Bit Manipulation	6/9/2023
0139	Word Break	Java	Medium	Array, Hash Table, String, Dynamic Programming, Trie, Memoization	9/2/2023	A really good example of Dynamic Programming. Worth a try!
0141	Linked List Cycle	Java	Easy	Hash Table, Linked List, Two Pointers	5/28/2023
0144	Binary Tree Preorder Traversal	Java	Easy	Stack, Tree, Depth-First Search, Binary Tree	8/29/2023
0145	Binary Tree Postorder Traversal	Java	Easy	Stack, Tree, Depth-First Search, Binary Tree	8/29/2023
0153	Find Minimum in Rotated Sorted Array	Java	Medium	Array, Binary Search	6/23/2023
0160	Intersection of Two Linked Lists	Java	Easy	Hash Table, Linked List, Two Pointers	5/28/2023
0162	Find Peak Element	Java	Medium	Array, Binary Search	9/27/2024	这道题目如果不限制时间复杂度为 O(log n) 其实难度不大，可以直接用循环 - 如果当前的值比后一个值小继续往后遍历，否则返回当前的索引。 但是加上了时间复杂度的限制为 O(log n) 后，我们需要考虑如何运用二分法来求解：比较当前值和后一个值的大小，如果当前大于后一个的值，说明有一个 peak 在当前或其前面，因此将右指针置为当前索引；如果当前小于后一个的值，说明有一个 peak 在后一个或其后面，因此将左指针置为当前索引 + 1。一直重复上述过程，直到左右指针重复，返回左指针所指向的索引的【注意不是返回中间指针】。
0167	Two Sum II - Input Array Is Sorted	Java	Medium	Array, Two Pointers, Binary Search	9/27/2024	典型二分法。
0168	Excel Sheet Column Title	Java	Easy	Math, String	5/29/2023	A little bit tricky!
0169	Majority Element	Java	Easy	Array, Hash Table, Divide and Conquer, Sorting	5/29/2023
0171	Excel Sheet Column Number	Java	Easy	Math, String	5/29/2023	The inverse process of 0168.
0175	Combine Two Tables	MySQL	Easy	Database	5/31/2023
0181	Employees Earning More Than Their Managers	MySQL	Easy	Database	5/31/2023	The efficiency of my solution is so low.
0182	Duplicate Emails	MySQL	Easy	Database	6/1/2023
0183	Customers Who Never Order	MySQL	Easy	Database	9/19/2023
0189	Rotate Array	Java	Medium	Array, Math, Two Pointers	6/18/2023
0195	Tenth Line	Bash	Easy	Shell	10/10/2023	Do not write spaces around the assignment sign, otherwise it would run into error.
0198	House Robber	Java	Medium	Array, Dynamic Programming	6/23/2023	The method of dynamic programming is really subtle.
0200	Number of Islands	Java	Medium	Array, Depth-First Search, Breadth-First Search, Union Find, Matrix	10/11/2024	这题注意数组是 char 类型而不是 int 类型，在编程的时候容易犯错去和 int 类型的数值判断了，需要特别注意。 相关题目：0463 Island Perimeter, 0695 Max Area of Island, 0827 Making A Large Island.
0202	Happy Number	Java	Easy	Math, Hash Table, Two Pointers	6/5/2023	A little bit tricky about how to record the loop.
0205	Isomorphic Strings	Java	Easy	String, Hash Table	5/31/2023
0206	Reverse Linked List	Java	Easy	Linked List, Recursion	6/14/2023	A typical link list problem (reverse the link list), worth a try!
0207	Course Schedule	Java	Medium	Depth-First Search, Breadth-First Search, Graph, Topological Sort	10/13/2024	这个题目使用的策略是拓扑排序，核心在于记录每门课程的入度和前置课和后置课之间的关系。将入度为 0 的课程存到一个队列里，之后每次找入度为 0 的课程，找到后将这门课从队列移除，并将其后置课程的入度减 1，如果有新的课程入度为 0，则将这门课添加到队列中，重复以上步骤直至队列为空。当队列为空时，所有的课程都从队列中出队了，说明不会有相互依赖的情况出现。
0209	Minimum Size Subarray Sum	Java	Medium	Array, Binary Search, Sliding Window, Prefix Sum	9/27/2024	方法一：sliding window 方法，设置两个 pointer，一个指向 start，一个指向 end。固定 start，移动 end，当 start 到 end 的值初次大于等于 target 时，继续start 到 end 之间的数字个数 count，start 向后移，重复之前步骤，找到最小的 count。
0210	Course Schedule II	Java	Medium	Depth-First Search, Breadth-First Search, Graph, Topological Sort	10/13/2024	这个题目是 0207 Course Schedule 的变式，只需要多记录一个访问的顺序而已。如果掌握了 0207，这个问题就没有什么难度。
0215	Kth Largest Element in an Array	Java	Medium	Array, Divide and Conquer, Sorting, Heap (Priority Queue), Quickselect	9/8/2023	Usually, problems with the form of asking the k-th largest or smallest number should utilize the structure of priority queue.
0217	Contains Duplicate	Java	Easy	Array, Hash Table, Sorting	6/1/2023	Really interesting when you are trying to find a better solution.
0219	Contains Duplicate II	Java	Easy	Array, Hash Table, Sliding Window	8/29/2023
0222	Count Complete Tree Nodes	Java	Easy	Binary Search, Bit Manipulation, Tree, Binary Tree	9/27/2024	这个题目如果不利用完全二叉树的性质解题，仅仅通过递归每个结点就可以求解。但是这种解法效率不高 - 时间复杂度为 O(n)。 如果利用完全二叉树的性质解题可以提高效率，对于一个节点 root，如果其左子树的高度比右子树大，那么可算出除了左子树的结点个数为 (2^rightHeight - 1) + 1 = 2^rightHeight【+1是加上 root 结点】；而如果左右子树高度一样，则可以计算出除了右子树的结点个数为 (2^leftHeight - 1) + 1 = 2^leftHeight【道理类似前面所说】。这个方法的时间复杂度为 O(log^2 n)。参考 此 leetcode 题解。
0226	Invert Binary Tree	Java	Easy	Tree, Depth-First Search, Breadth-First Search, Binary Tree	6/5/2023	I am weak at solving problems related to Tree. It took me a relatively long time to solve this problem.
0228	Summary Ranges	Java	Easy	Array	6/13/2023	It takes me a while to figure out the problem.
0231	Power of Two	Java	Easy	Math, Bit Manipulation, Recursion	6/4/2023	It is easy to misunderstand the problem, so more attention is needed.
0234	Palindrome Linked List	Java	Easy	Linked List, Two Pointers, Stack, Recursion	6/14/2023
0238	Product of Array Except Self	Java	Medium	Array, Prefix Sum	6/19/2023	More like a easy problem than a medium one.
0242	Valid Anagram	Java	Easy	String, Hash Table, Sorting	5/31/2023
0257	Binary Tree Paths	Java	Easy	String, Backtracking, Tree	5/31/2023	Not that easy, worth a try.
0258	Add Digits	Java	Easy	Math, Simulation, Number Theory	5/31/2023	Very interesting! Once you find the pattern, it can be solved with three lines of code.
0263	Ugly Number	Java	Easy	Math	8/29/2023
0264	Ugly Number II	Java	Medium	Hash Table, Math, Dynamic Programming, Heap (Priority Queue)	9/28/2024	这道题目目前采用的是最小堆的方式来解题的：首先将 1 添加到最小堆里，取最小堆的 root，然后依次将最小堆里 root 的 2 倍，3 倍和 5 倍添加进来（如果要添加的数已经存在就不添加进树里了），重复以上步骤直到取出了 n 个数。但是这个解法效率不高，后续待补充动态规划的解法。
0268	Missing Number	Java	Easy	Array, Hash Table, Math, Binary Search, Bit Manipulation, Sorting	6/9/2023
0278	First Bad Version	Java	Easy	Binary Search, Interactive	6/10/2023	A little bit tricky to use proper data structure.
0283	Move Zeroes	Java	Easy	Array, Two Pointers	6/9/2023
0290	Word Pattern	Java	Easy	Hash Table, String	8/30/2023	Pay attention to the problem description. It is a little bit tricky.
0292	Nim Game	Java	Easy	Math, Brainteaser, Game Theory	8/31/2023	Not worth trying. The idea is to find the pattern of the solution, like when n = 1, 2, 3, 4 and n = 5, 6, 7, 8 and so on and so forth.
0303	Range Sum Query - Immutable	Java	Easy	Array, Design, Prefix Sum	8/31/2023
0310	Minimum Height Trees	Java	Medium	Depth-First Search, Breadth-First Search, Graph, Topological Sort	10/13/2024	这个题难点在于如何找到它和拓扑排序之间的关联，也就是为什么要一层一层去掉度数为 1 的结点（这个结论的证明不是太理解，有兴趣可以参考 力扣中国区官方题解 的证明）。 如果理解了其解题逻辑，写代码其实并不是最难的部分，多花一点时间还是可以写出来的，注意边界情况即可。
0326	Power of Three	Java	Easy	Math, Recursion	8/31/2023
0338	Counting Bits	Java	Easy	Dynamic Programming, Bit Manipulation	5/30/2023	Not that easy, worth a try.
0344	Reverse String	Java	Easy	String, Two Pointers	6/6/2023
0345	Reverse Vowels of a String	Java	Easy	String, Two Pointers	6/11/2023
0347	Top K Frequent Elements	Java	Medium	Array, Hash Table, Divide and Conquer, Sorting, Heap (Priority Queue), Bucket Sort, Counting, Quickselect	6/15/2023
0383	Ransom Note	Java	Easy	Hash Table, String, Counting	6/14/2023
0384	Shuffle an Array	Java	Medium	Array, Math, Randomized	10/13/2024	这道题目的难点在于如何使得元素的 shuffle 是随机的，这个其实是蓄水池算法的微缩版，可以参考博客园的这篇文章：谈谈游戏中的随机算法。 简单来说，就是遍历 nums 数组，将第 i 个元素换到索引值 [i, nums.length - 1] 这个范围里随机的位置上即可（即相当于这 n 个元素有 n! 种排列方式）。
0392	Is Subsequence	Java	Easy	Dynamic Programming, Two Pointers, String	5/30/2023
0409	Longest Palindrome	Java	Easy	Hash Table, String, Greedy	7/9/2023
0412	Fizz Buzz	Java	Easy	Math, String, Simulation	6/1/2023
0414	Third Maximum Number	Java	Easy	Array, Sorting	6/1/2023
0415	Add Strings	Java	Easy	Math, String, Simulation	6/1/2023
0435	Non-overlapping Intervals	Java	Medium	Array, Dynamic Programming, Greedy, Sorting	9/25/2024	解题思想：贪心算法，按照 interval 的结束时间来排序，在选择的时候，优先早结束的 interval，因为它会为后面的 interval 留更多的时间（贪心算法）。
0436	Find Right Interval	Java	Medium	Array, Binary Search, Sorting	9/23/2024	第一种解法：暴力解法，双重循环，时间复杂度为 O(n^2) 第二种解法：二分搜索法，时间复杂度优化为 O(nlogn)
0455	Assign Cookies	Java	Easy	Array, Two Pointers, Greedy, Sorting	7/9/2023
0463	Island Perimeter	Java	Easy	Array, Depth-First Search, Breadth-First Search, Matrix	10/11/2024	这道题目看似简单，如果没有弄清原理其实也不算简单。计算岛屿的周长的核心思路是当一个点的上下左右任意相邻的一边的点越界或为 0 时，这条边才为周长的一部分，否则只是岛屿内部的一条边而已。参考 此解。 相关题目：0200 Number of Islands, 0695 Max Area of Island, 0827 Making A Large Island.
0501	Find Mode in Binary Search Tree	Java	Easy	Tree, Depth-First Search, Binary Search Tree, Binary Tree	7/18/2024	Takeaway of solution 2 with O(1) space complexity: 利用 BST 的有序性，中序遍历 BST 可以得到有序数列，因此题目可以转换为在一个有序数列里找众数。
0509	Fibonacci Number	Java	Easy	Math, Dynamic Programming, Recursion, Memoization	6/14/2023
0530	Minimum Absolute Difference in BST	Java	Easy	Tree, Depth-First Search, Breadth-First Search, Binary Search Tree, Binary Tree	6/14/2023	This problem is almost the same as 0783. Try to make full use of the features of BST (Binary Search Tree)!
0543	Diameter of Binary Tree	Java	Easy	Tree, Depth-First Search, Binary Tree	6/11/2023	A little bit tricky, not that easy! Worth a try.
0547	Number of Provinces	Java	Medium	Depth-First Search, Breadth-First Search, Union Find, Graph	6/19/2023 10/14/2024	时隔一年再次做这道题，这次的解法更简洁了（利用 DFS 解题）。
0561	Array Partition	Java	Easy	Array, Greedy, Sorting, Counting Sort	7/3/2023
0584	Find Customer Referee	MySQL	Easy	Database	6/14/2023
0595	Big Countries	MySQL	Easy	Database	6/14/2023
0605	Can Place Flowers	Java	Easy	Array, Greedy	6/9/2023
0680	Valid Palindrome II	Java	Easy	Two Pointers, String, Greedy	7/10/2023
0695	Max Area of Island	Java	Medium	Array, Depth-First Search, Breadth-First Search, Union Find, Matrix	10/11/2024	相关题目：0200 Number of Islands, 0463 Island Perimeter, 0827 Making A Large Island.
0703	Kth Largest Element in a Stream	Java	Easy	Tree, Design, Binary Search Tree, Heap (Priority Queue), Binary Tree, Data Stream	6/13/2023
0704	Binary Search	Java	Easy	Array, Binary Search	6/8/2023
0705	Design HashSet	Java	Easy	Array, Hash Table, Linked List, Design, Hash Function	6/9/2023
0744	Find Smallest Letter Greater Than Target	Java	Easy	Array, Binary Search	6/9/2023
0746	Min Cost Climbing Stairs	Java	Easy	Array, Dynamic Programming	6/27/2023
0783	Minimum Distance Between BST Nodes	Java	Easy	Tree, Depth-First Search, Breadth-First Search, Binary Search Tree, Binary Tree	6/14/2023	This problem is almost the same as 0530. Try to make full use of the features of BST (Binary Search Tree)!
0807	Max Increase to Keep City Skyline	Java	Medium	Array, Greedy, Matrix	7/10/2023	The method to calculate the maximum value in each row and each column with only one double nested for loop is fantastic.
0827	Making A Large Island	Java	Hard	Array, Depth-First Search, Breadth-First Search, Union Find, Matrix	10/11/2024	相关题目：0200 Number of Islands, 0463 Island Perimeter, 0695 Max Area of Island.
0860	Lemonade Change	Java	Easy	Array， Greedy	7/9/2023
0873	Length of Longest Fibonacci Subsequence	Java	Medium	Array，Hash Table，Dynamic Programming	2/1/2024
0874	Walking Robot Simulation	Java	Medium	Array, Hash Table, Simulation	2/1/2024	这个题目看似简单，实际难点在于obstacles的判断：本题必须用HashSet而非List来进行obstacles的判定，因为hashset的查找效率更快，使用List会导致超时。另外就是如何将二维的坐标转化为一维的数据：这里是根据xi和yi的范围来实现的，详细实现参看代码。
0876	Middle of the Linked List	Java	Easy	Linked List, Two Pointers	6/14/2023
0877	Stone Game	Java	Medium	Array, Math, Dynamic Programming, Game Theory	1/30/2024
0880	Decoded String at Index	Python	Medium	String, Stack	3/25/2024	This problem seems easy to solve, but how to minimize the memory is the key issue. The basic idea is to record how the total length would be influenced when one character is read, then retrodict which character would be at the given index.
0881	Boats to Save People	Java	Medium	Array, Two Pointers, Greedy, Sorting	3/26/2024	This problem seems kind of tricky at first, but I found that I got the problem wrong twice. We need to take care of the requirement that each boat can carry at most two people at the same time.
0938	Range Sum of BST	Java	Easy	Tree, Depth-First Search, Binary Search Tree, Binary Tree	7/18/2024
0942	DI String Match	Java	Easy	Array, Two Pointers, String, Greedy	7/3/2023	The solution which used two pointers is really subtle.
0976	Largest Perimeter Triangle	Java	Easy	Array, Math, Greedy, Sorting	7/9/2023
0977	Squares of a Sorted Array	Java	Easy	Array, Two Pointers, Sorting	6/5/2023
0986	Interval List Intersections	Java	Medium	Array, Two Pointers	9/22/2024	学到的内容： 1. 创建 int[] 类型的ArrayList 2. 求两个区间的交集：如果两个区间的起点的最大值小于等于终点的最小值，则说明有交集 3. int[] 类型的 List 转数组的方法：res.toArray(new int[0][])
1005	Maximize Sum Of Array After K Negations	Java	Easy	Array, Greedy, Sorting	7/9/2023	Note that in the problem description we may choose the same index multiple times.
1013	Partition Array Into Three Parts With Equal Sum	Java	Easy	Array, Greedy	7/10/2023
1025	Divisor Game	Java	Easy	Math, Dynamic Programming, Brainteaser, Game Theory	6/29/2023	It depends heavily on mathematical induction. Not worth giving it a shot.
1027	Longest Arithmetic Subsequence	Java	Medium	Array, Hash Table, Binary Search, Dynamic Programming	6/26/2023
1071	Greatest Common Divisor of Strings	Java	Easy	Math, String	6/10/2023	A little tricky, not that easy! It is easy to misunderstand the problem.
1091	Shortest Path in Binary Matrix	Java	Medium	Array, Breadth-First Search, Matrix	6/18/2023	The idea of the solution is similar to Bellman-Ford /algorithm. And the double-nested loop is the important part to simplify the solution which has eight "if" branches.
1137	N-th Tribonacci Number	Java	Easy	Math, Dynamic Programming, Memoization	6/28/2023
1161	Maximum Level Sum of a Binary Tree	Java	Medium	Tree, Depth-First Search, Breadth-First Search, Binary Tree	6/19/2023	Make sure you understand what the problem asks for first before starting to solve it.
1217	Minimum Cost to Move Chips to The Same Position	Java	Easy	Array, Math, Greedy	7/7/2023	Pay attention to the cost part.
1221	Split a String in Balanced Strings	Java	Easy	String, Greedy, Counting	7/2/2023
1232	Check If It Is a Straight Line	Java	Easy	Array, Math, Geometry	6/8/2023
1318	Minimum Flips to Make a OR b Equal to c	Java	Medium	Bit Manipulation	6/22/2023
1323	Maximum 69 Number	Java	Easy	Math, Greedy	7/2/2023
1337	The K Weakest Rows in a Matrix	Java	Easy	Array, Binary Search, Sorting, Heap (Priority Queue), Matrix	1/29/2024	The first method might be more efficient, but a lot can be learnt from the second method---the sorting of map.
1351	Count Negative Numbers in a Sorted Matrix	Java	Easy	Array, Binary Search, Matrix	6/12/2023	The solution 2 is so ingenious a solution!
1353	Maximum Number of Events That Can Be Attended	Java	Medium	Array, Greedy, Sorting, Heap (Priority Queue)	9/25/2024	这题主要是采用贪心算法，每往后遍历一天，先清除过期的 event，然后添加今天开始的 event，最后选择一个会议 - 结束时间最早的会议（贪心）。
1403	Minimum Subsequence in Non-Increasing Order	Java	Easy	Array, Greedy, Sorting	7/10/2023
1480	Running Sum of 1d Array	Java	Easy	Array, Prefix Sum	6/9/2023
1491	Average Salary Excluding the Minimum and Maximum Salary	Java	Easy	Array, Sorting	6/1/2023
1502	Can Make Arithmetic Progression From Sequence	Java	Easy	Array, Sorting	6/8/2023
1517	Find Users With Valid E-Mails	Java	Easy	Database	6/30/2023
1518	Water Bottles	Java	Easy	Math, Simulation	6/30/2023
1523	Count Odd Numbers in an Interval Range	Java	Easy	Math	6/30/2023
1528	Shuffle String	Java	Easy	Array, String	6/30/2023	The method of sorting a 2d array by one of the two dimensions is a really important method to solve problems.
1539	Kth Missing Positive Number	Java	Easy	Array, Binary Search	1/28/2024
1603	Design Parking System	Java	Easy	Design, Simulation, Counting	6/14/2023
1641	Count Sorted Vowel Strings	Java	Medium	Math, Dynamic Programming, Combinatorics	7/13/2023
1646	Get Maximum in Generated Array	Java	Easy	Array, Dynamic Programming, Simulation	6/28/2023
1689	Partitioning Into Minimum Number Of Deci-Binary Numbers	Java	Medium	String, Greedy	7/10/2023
1710	Maximum Units on a Truck	Java	Easy	Array, Greedy, Sorting	7/3/2023
1732	Find the Highest Altitude	Java	Easy	Array, Prefix Sum	6/21/2023
1736	Latest Time by Replacing Hidden Digits	Java	Easy	String, Greedy	7/9/2023
1768	Merge Strings Alternately	Java	Easy	Two Pointers, String	6/14/2023
1822	Sign of the Product of an Array	Java	Easy	Array, Math	6/1/2023
1827	Minimum Operations to Make the Array Increasing	Java	Easy	Math, Greedy	7/2/2023
1903	Largest Odd Number in String	Java	Easy	Math, String, Greedy	7/9/2023
1974	Minimum Time to Type Word Using Special Typewriter	Java	Easy	String, Greedy	7/3/2023
2027	Minimum Moves to Convert String	Java	Easy	String, Greedy	7/9/2023
2078	Two Furthest Houses With Different Colors	Java	Easy	Array, Greedy	7/8/2023	The idea is subtle.
2089	Find Target Indices After Sorting Array	Java	Easy	Array, Binary Search, Sorting	1/27/2024
2144	Minimum Cost of Buying Candies With Discount	Java	Easy	Array, Greedy, Sorting	7/8/2023
2160	Minimum Sum of Four Digit Number After Splitting Digits	Java	Easy	Math, Greedy, Sorting	7/2/2023
2215	Find the Difference of Two Arrays	Java	Easy	Array, Hash Table	6/1/2023	The method I used is just like the most original one, which has a low efficiency.
2224	Minimum Number of Operations to Convert Time	Java	Easy	String, Greedy	7/8/2023
2259	Remove Digit From Number to Maximize Result	Java	Easy	String, Greedy, Enumeration	7/9/2023
2335	Minimum Amount of Time to Fill Cups	Java	Easy	Array, Greedy, Sorting, Heap (Priority Queue)	7/9/2023	The usage of "diff / 2 + diff % 2 = (diff + 1) / 2" is really subtle.
2352	Equal Row and Column Pairs	Java	Medium	Array, Hash Table, Matrix, Simulation	6/16/2023
2357	Make Array Zero by Subtracting Equal Amounts	Java	Easy	Array, Hash Table, Greedy, Sorting, Heap (Priority Queue), Simulation	7/3/2023	It is my first time to use heap (Priority Queue).
2383	Minimum Hours of Training to Win a Competition	Java	Easy	Array, Greedy	7/11/2023	The "initialExperience <= experience[i]" part is a little bit tricky.
2389	Longest Subsequence With Limited Sum	Java	Easy	Array, Binary Search, Greedy, Sorting, Prefix Sum	7/4/2023	The second solution which adopts binary search and the third solution which uses the built-in function Arrays.binarySearch are worth learning.
2566	Maximum Difference by Remapping a Digit	Java	Easy	Math, Greedy	7/8/2023
2578	Split With Minimum Sum	Java	Easy	Math, Greedy, Sorting	7/7/2023
2591	Distribute Money to Maximum Children	Java	Easy	Array, Greedy	6/12/2023	A little tricky, not that easy. It is important to think it thoroughly before you start coding.
2600	K Items With the Maximum Sum	Java	Easy	Math, Greedy	7/8/2023
2656	Maximum Sum With Exactly K Elements	Java	Easy	Array, Greedy	6/6/2023
2696	Minimum String Length After Removing Substrings	Java	Easy	String, Stack, Simulation	6/6/2023	Involved with the use of the stack structure.
2706	Buy Two Chocolates	Java	Easy	Array, Sorting	6/10/2023	Try to find a better solution to have some fun.
2708	Maximum Strength of a Group	Java	Medium	Array, Greedy, Recursion, Sorting	6/22/2023
2710	Remove Trailing Zeros From a String	Java	Easy	String	6/10/2023
2717	Semi-Ordered Permutation	Java	Easy	Array, Simulation	6/10/2023	A little interesting to discuss the two cases.
2718	Sum of Matrix After Queries	Java	Medium	Array, Hash Table	6/24/2023	The solution is really subtle about the starting value of the loop, cuz the latter assignment will overwrite the former assignments. So it is a good way to loop reveresely.
2729	Check if The Number is Fascinating	Java	Easy	Hash Table, Math	6/11/2023
2733	Neither Minimum nor Maximum	Java	Easy	Array, Sorting	6/12/2023	Pay more attention to the problem statement, especially the keyword any. Have fun to find better solution than ever before.
2739	Total Distance Traveled	Java	Easy	Math, Simulation	6/23/2023	The second solution is really subtle with just one line of code.
2824	Count Pairs Whose Sum is Less than Target	Java	Easy	Array, Two Pointers, Binary Search, Sorting	1/27/2024