环形链表 II

[Sunny Fu]

[Li Xiangyue]

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var detectCycle = function(head) {
    let cur = head;
    while(cur){
        if(cur.dirty){
            return cur;
        }
        cur.dirty = true; //打个标记
        cur = cur.next;
    }
    return null
};