feat(linked-list): add 21_merge_two_sorted_lists.rs

Author: whisperpine

README.md

@@ -130,3 +130,4 @@ so that you can use `just tf TEST` command to test.
 | Problem | Difficulty | Solution |
 | - | - | - |
 | 206. Reverse Linked List | Easy | [206_reverse_linked_list.rs](./tests/206_reverse_linked_list.rs) |
+| 21. Merge Two Sorted Lists | Easy | [21_merge_two_sorted_lists.rs](./tests/21_merge_two_sorted_lists.rs) |

tests/21_merge_two_sorted_lists.rs

@@ -0,0 +1,53 @@
+// 21. Merge Two Sorted Lists
+// https://leetcode.com/problems/merge-two-sorted-lists/description/
+// Topics: Linked List.
+// Difficulty: Easy.
+
+#[test]
+fn test_21_merge_two_sorted_lists() {}
+
+#[derive(Debug)]
+pub struct Solution;
+
+// ---------------------------------
+// copy to leetcode starts from here
+// ---------------------------------
+
+// Definition for singly-linked list.
+#[derive(PartialEq, Eq, Clone, Debug)]
+pub struct ListNode {
+    pub val: i32,
+    pub next: Option<Box<ListNode>>,
+}
+
+// impl ListNode {
+//     #[inline]
+//     fn new(val: i32) -> Self {
+//         ListNode { next: None, val }
+//     }
+// }
+
+impl Solution {
+    pub fn merge_two_lists(
+        list1: Option<Box<ListNode>>,
+        list2: Option<Box<ListNode>>,
+    ) -> Option<Box<ListNode>> {
+        match (list1, list2) {
+            (None, None) => None,
+            (Some(n), None) | (None, Some(n)) => Some(n),
+            (Some(l1), Some(l2)) => {
+                if l1.val >= l2.val {
+                    Some(Box::new(ListNode {
+                        val: l2.val,
+                        next: Solution::merge_two_lists(Some(l1), l2.next),
+                    }))
+                } else {
+                    Some(Box::new(ListNode {
+                        val: l1.val,
+                        next: Solution::merge_two_lists(l1.next, Some(l2)),
+                    }))
+                }
+            }
+        }
+    }
+}