Leetcode•Oct 14, 2026

Reorder List

Hazrat Ali

Leetcode

You are given the head of a singly linked-list. The list can be represented as:

L0 → L1 → … → Ln - 1 → Ln

Reorder the list to be on the following form:

L0 → Ln → L1 → Ln - 1 → L2 → Ln - 2 → …

You may not modify the values in the list's nodes. Only nodes themselves may be changed.

 

Example 1:

Input: head = [1,2,3,4]
Output: [1,4,2,3]

Example 2:

Input: head = [1,2,3,4,5]
Output: [1,5,2,4,3]

Solution
var reorderList = function(head) {
    if (!head || !head.next) return;

    let slow = head;
    let fast = head;

    while (fast.next && fast.next.next) {
        slow = slow.next;
        fast = fast.next.next;
    }

    let prev = null;
    let curr = slow.next;
    slow.next = null;

    while (curr) {
        let nextNode = curr.next;
        curr.next = prev;
        prev = curr;
        curr = nextNode;
    }
        let first = head;
    let second = prev;

    while (second) {
        let temp1 = first.next;
        let temp2 = second.next;

        first.next = second;
        second.next = temp1;

        first = temp1;
        second = temp2;
    }
};
 

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