/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
    node=head;
    while(node){
        //初次用到node.isVisit = undefined
        if(node.isVisit){
            return true; 
        }else{
            node.isVisit=true;
        }
        node=node.next;
    }
    return false;
};