linked-list-cycle-II.js

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var detectCycle = function (head) {
    var fast = head, slow = head;

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

        if (fast === slow) {
            fast = head;
            while (fast !== slow) {
                fast = fast.next;
                slow = slow.next;
            }

            return slow;
        }
    }

    return null;
};