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

/**
 * 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;
};