Lintcode 102.带环链表

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只要设置两个指针，称为快慢指针，当链表没有环的时候快指针会走到null，当链表有环的时候快指针早晚会追上慢指针的。

 

AC代码：

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param head: The first node of linked list.
     * @return: True if it has a cycle, or false
     */
    public boolean hasCycle(ListNode head) {
        
        if(head==null || head.next==null) return false;
        
        ListNode fast=head.next.next;
        ListNode slow=head.next;
        
        while(true){
            if(fast==null || fast.next==null){
                return false;
            }else if(fast==slow || fast.next==slow){
                return true;
            }else{
                fast=fast.next.next;
                slow=slow.next;
            }
        }
    }
}

 

 

题目来源： http://www.lintcode.com/zh-cn/problem/linked-list-cycle/