LeetCode - 25 - Reverse Nodes in k-Group

 /**

 * 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) {
    if (head == null) {
        return head;
    }
    var pointer = head;
    var stack = [];
    while (pointer != null && stack.length < k ) {
        var temp = pointer;
        pointer = pointer.next;
        temp.next = null;
        stack.push(temp);
    }
    if (stack.length < k) {
        var stack2 = [];
        while (stack.length > 0) {
            stack2.push(stack.pop());
        }
        stack = stack2;
    }
    var nextElements = reverseKGroup(pointer, k);
    var newHead = stack.pop();
    var pointer2 = newHead;
    while (stack.length > 0) {
        pointer2.next = stack.pop();
        pointer2 = pointer2.next;
    }
    pointer2.next = nextElements;
    return newHead;
};