141. Linked List Cycle

题目:

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

链接： http://leetcode.com/problems/linked-list-cycle/

题解：

链表检测是否有环，使用快慢指针。

Time Complexity - O(n)， Space Complexity - O(1)。

public class Solution {

    public boolean hasCycle(ListNode head) {

        if(head == null || head.next == null)

            return false;

        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;

    }

}

 

测试:

Reference:

http://wenku.baidu.com/view/c8f09f17376baf1ffc4fad6e.html?re=view