Add solution #1721

Author: JoshCrozier

README.md

@@ -1,4 +1,4 @@
-# 1,499 LeetCode solutions in JavaScript
+# 1,500 LeetCode solutions in JavaScript
 
 [https://leetcodejavascript.com](https://leetcodejavascript.com)
 
@@ -1326,6 +1326,7 @@
 1718|[Construct the Lexicographically Largest Valid Sequence](./solutions/1718-construct-the-lexicographically-largest-valid-sequence.js)|Medium|
 1719|[Number Of Ways To Reconstruct A Tree](./solutions/1719-number-of-ways-to-reconstruct-a-tree.js)|Hard|
 1720|[Decode XORed Array](./solutions/1720-decode-xored-array.js)|Easy|
+1721|[Swapping Nodes in a Linked List](./solutions/1721-swapping-nodes-in-a-linked-list.js)|Medium|
 1726|[Tuple with Same Product](./solutions/1726-tuple-with-same-product.js)|Medium|
 1732|[Find the Highest Altitude](./solutions/1732-find-the-highest-altitude.js)|Easy|
 1748|[Sum of Unique Elements](./solutions/1748-sum-of-unique-elements.js)|Easy|

solutions/1721-swapping-nodes-in-a-linked-list.js

@@ -0,0 +1,42 @@
+/**
+ * 1721. Swapping Nodes in a Linked List
+ * https://leetcode.com/problems/swapping-nodes-in-a-linked-list/
+ * Difficulty: Medium
+ *
+ * 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).
+ */
+
+/**
+ * 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) {
+  let firstNode = head;
+  for (let i = 1; i < k; i++) {
+    firstNode = firstNode.next;
+  }
+
+  let slow = head;
+  let secondNode = firstNode.next;
+  while (secondNode) {
+    slow = slow.next;
+    secondNode = secondNode.next;
+  }
+
+  const temp = firstNode.val;
+  firstNode.val = slow.val;
+  slow.val = temp;
+
+  return head;
+};