Add solution #25

Author: JoshCrozier

README.md

@@ -27,6 +27,7 @@
 22|[Generate Parentheses](./0022-generate-parentheses.js)|Medium|
 23|[Merge k Sorted Lists](./0023-merge-k-sorted-lists.js)|Hard|
 24|[Swap Nodes in Pairs](./0024-swap-nodes-in-pairs.js)|Medium|
+25|[Reverse Nodes in k-Group](./0025-reverse-nodes-in-k-group.js)|Hard|
 26|[Remove Duplicates from Sorted Array](./0026-remove-duplicates-from-sorted-array.js)|Easy|
 27|[Remove Element](./0027-remove-element.js)|Easy|
 28|[Implement strStr()](./0028-implement-strstr.js)|Easy|

solutions/0025-reverse-nodes-in-k-group.js

@@ -0,0 +1,49 @@
+/**
+ * 25. Reverse Nodes in k-Group
+ * https://leetcode.com/problems/reverse-nodes-in-k-group/
+ * Difficulty: Hard
+ *
+ * Given the head of a linked list, reverse the nodes of the list k at a time,
+ * and return the modified list.
+ *
+ * k is a positive integer and is less than or equal to the length of the linked
+ * list. If the number of nodes is not a multiple of k then left-out nodes, in
+ * the end, should remain as it is.
+ *
+ * You may not alter the values in the list's nodes, only nodes themselves may
+ * be changed.
+ */
+
+/**
+ * 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 reverseKGroup = function(head, k) {
+  const result = new ListNode(null, head);
+  const stack = [];
+  let tail = result;
+
+  while (head) {
+    for (let i = 0; i < k && head; i++) {
+      stack.push(head);
+      head = head.next;
+    }
+    if (stack.length === k) {
+      while (stack.length) {
+        tail.next = stack.pop();
+        tail = tail.next;
+      }
+      tail.next = head;
+    }
+  }
+
+  return result.next;
+};