25. Reverse Nodes in k-Group

Reverse the nodes of a linked list k at a time. If the number of remaining nodes is less than k, leave them as is. Only node links may change, not values.

Keep a pointer to the node before the current group. Check that k nodes remain, reverse them with the usual prev/curr loop, then splice the reversed group back between its neighbours.

1function reverseKGroup(head: ListNode | null, k: number): ListNode | null {
2  const dummy = new ListNode(0, head);
3  let before = dummy;
4  while (true) {
5    let kth: ListNode | null = before;
6    for (let i = 0; i < k && kth; i++) kth = kth.next;
7    if (!kth) break;
8    const after = kth.next, first = before.next!;
9    let prev = after, curr: ListNode | null = first;
10    while (curr !== after) { const nx: ListNode | null = curr!.next; curr!.next = prev; prev = curr; curr = nx; }
11    before.next = kth;
12    before = first;
13  }
14  return dummy.next;
15}

k = 3

list

before

→

→ ∅

A dummy head makes the first group like any other.

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