[LeetCode] 413. Arithmetic Slices

[frdmu]

An integer array is called arithmetic if it consists of at least three elements and if the difference between any two consecutive elements is the same.

• For example, [1,3,5,7,9], [7,7,7,7], and [3,-1,-5,-9] are arithmetic sequences.
  Given an integer array nums, return the number of arithmetic subarrays of nums.

A subarray is a contiguous subsequence of the array.

 

Example 1:

Input: nums = [1,2,3,4]
Output: 3
Explanation: We have 3 arithmetic slices in nums: [1, 2, 3], [2, 3, 4] and [1,2,3,4] itself.

Example 2:

Input: nums = [1]
Output: 0

Constraints:

• 1 <= nums.length <= 5000
• -1000 <= nums[i] <= 1000

 

解法：
前缀数组+滑动窗口。举两个例子：

代码如下：

class Solution {
public:
    int numberOfArithmeticSlices(vector<int>& nums) {
        int n = nums.size();
        vector<int> tmp(n-1);
        
        for (int i = 0; i < n-1; i++) {
            tmp[i] = nums[i+1] - nums[i];
        }
        int left = 0, res = 0;
        for (int right = 0; right < n; right++) {
            if (right == n-1 || tmp[right] != tmp[left]) {
                int dis = right - left;
                res += dis * (dis-1) / 2;
                left = right;       
            }
        }

        return res;
    }
};