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【Leetcode】Linked List Cycle

一、原题

Linked List Cycle

 

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

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

二、分析

选用两个指针扫描链表,一个速度快,一个速度慢,若两个指针相遇,说明有环。

三、代码(java)

/**
 * 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) {
       boolean result=false;
		if(head==null||head.next==null){}
		else{
		    ListNode fast,slow;
	        slow=head;
		    fast=head;
		    while(fast!=null)
		   {
			  if(fast.next==null||fast==null)
				  break;
			  else{
				  fast=fast.next.next;
			      slow=slow.next;
			      }
			  if(fast==slow){
				 result=true;
				 break;
			  }
		   }
		}	
		return result;
    }
}

关于快慢指针相遇表明链表有环的证明:

http://umairsaeed.com/2011/06/23/finding-the-start-of-a-loop-in-a-circular-linked-list/。

比较直观的解释,如果两个点在一个圆上跑,方向相同,只要速度不一样就一定会相遇,在直线上则不一定。


【Leetcode】Linked List Cycle