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

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var detectCycle = function(head) {
    if (!head || !head.next) return null;
    var slow = head;
    var fast = head;

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

    return null;
};