Solution.ts

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     val: number
 *     next: ListNode | null
 *     constructor(val?: number, next?: ListNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.next = (next===undefined ? null : next)
 *     }
 * }
 */

/**
 Do not return anything, modify head in-place instead.
 */
function reorderList(head: ListNode | null): void {
    let slow = head;
    let fast = head;

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

    let next = slow.next;
    slow.next = null;
    while (next != null) {
        [next.next, slow, next] = [slow, next, next.next];
    }

    let left = head;
    let right = slow;
    while (right.next != null) {
        const next = left.next;
        left.next = right;
        right = right.next;
        left.next.next = next;
        left = left.next.next;
    }
}