feat: solve No.1721

BaffinLee · BaffinLee · commit 2d39c69ab133 · 2023-05-15T23:58:39.000+08:00
diff --git a/1701-1800/1721. Swapping Nodes in a Linked List.md b/1701-1800/1721. Swapping Nodes in a Linked List.md
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+# 1721. Swapping Nodes in a Linked List
+
+- Difficulty: Medium.
+- Related Topics: Linked List, Two Pointers.
+- Similar Questions: Remove Nth Node From End of List, Swap Nodes in Pairs, Reverse Nodes in k-Group.
+
+## Problem
+
+You are given the `head` of a linked list, and an integer `k`.
+
+Return **the head of the linked list after **swapping** the values of the **`kth` **node from the beginning and the **`kth` **node from the end (the list is **1-indexed**).**
+
+ 
+Example 1:
+
+![](https://assets.leetcode.com/uploads/2020/09/21/linked1.jpg)
+
+```
+Input: head = [1,2,3,4,5], k = 2
+Output: [1,4,3,2,5]
+```
+
+Example 2:
+
+```
+Input: head = [7,9,6,6,7,8,3,0,9,5], k = 5
+Output: [7,9,6,6,8,7,3,0,9,5]
+```
+
+ 
+**Constraints:**
+
+
+	
+- The number of nodes in the list is `n`.
+	
+- `1 <= k <= n <= 105`
+	
+- `0 <= Node.val <= 100`
+
+
+
+## Solution
+
+```javascript
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val, next) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.next = (next===undefined ? null : next)
+ * }
+ */
+/**
+ * @param {ListNode} head
+ * @param {number} k
+ * @return {ListNode}
+ */
+var swapNodes = function(head, k) {
+    if (!head) {
+        return null;
+    }
+    let left = head, right = head;
+    for (let i = 1; i < k; i++) {
+        left = left.next;
+    }
+    let curr = left;
+    while (curr.next) {
+        curr = curr.next;
+        right = right.next;
+    }
+    let temp = left.val;
+    left.val = right.val;
+    right.val = temp;
+    return head;
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(1).
