25-Reverse-Nodes-In-K-Group.js

/**
 * https://leetcode.com/problems/reverse-nodes-in-k-group/
 * Time O(N) | Space O(N)
 * @param {ListNode} head
 * @param {number} k
 * @return {ListNode}
 */
 var reverseKGroup = function(head, k) {
    const sentinel = tail = new ListNode(0, head);

    while (true) {
        let [ start, last ]= moveNode(tail, k);
        if (!last) break;

        reverse([ start, tail.next, start ])

        const next = tail.next;
        tail.next = last;
        tail = next;
    }

    return sentinel.next;
};

const moveNode = (curr, k) => {
    const canMove = () => k && curr;
    while (canMove()) {
        curr = curr.next;
        k--;
    }

    return [ (curr?.next || null), curr ];
}

const reverse = ([ prev, curr, start ]) => {
    const isSame = () => curr === start;
    while (!isSame()) {
        const next = curr.next;
        curr.next = prev;

        prev = curr;
        curr = next;
    }
}