leetcode 141. 环形链表

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

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
    // 0
    let slow = head;
    let fast = head;
    let ret = false;
    // 1
    while(fast && fast.next !== null){
        fast = fast.next.next
        slow = slow.next;
        if(fast === slow){
            ret = true;
            break;
        }
    }
    // 2
    return ret;
};