Linked List Cycle

1.题目

给定一个链表，判断它是否有环。

给出 -21->10->4->5, tail connects to node index 1，返回 true

2.算法

假设有两个指针walker和runner,walker用一倍速度向前走，runner用2倍速度向前走，当两指针相遇时，返回true

    public boolean hasCycle(ListNode head) {          // write your code here        if (head == null)        {            return false;        }        ListNode walker = head;        ListNode runner = head;        while (runner != null && runner.next != null)        {            walker = walker.next;            runner = runner.next.next;            if (walker == runner)            {                return true;            }        }        return false;    }