1. var reverseKGroup = function(head, k) {
    2.     let a = head, b = head;
    3.     for (let i = 0; i < k; i++) {
    4.         if (b == null) return head;
    5.         b = b.next;
    6.     }
    7.     const newHead = reverse(a, b);
    8.     a.next = reverseKGroup(b, k);
    9.     return newHead;
    10. };
    11. function reverse(a, b) {
    12.     let prev = null, cur = a, nxt = a;
    13.     while (cur != b) {
    14.         nxt = cur.next;
    15.         cur.next = prev;
    16.         prev = cur;
    17.         cur = nxt;
    18.     }
    19.     return prev;
    20. }

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