leetcode141.环形链表

直接快慢指针，如果有环，那么快指针一定会在成环的起始点与慢指针相遇 

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode slow=head,fast=head;
        while(fast!=null&&fast.next!=null){
            fast=fast.next.next;
            slow=slow.next;
            if(slow==fast)
                return true;
        }
        return false;
    }
}