题目

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

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

方法

/**
 * 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) {
        if (head == null || head.next == null) {
    		return false;
    	}
    	ListNode first = head;
    	ListNode second = head;
    	while (second != null) {
    		second = second.next;
    		if (second != null) {
    			second = second.next;
    		} else {
    			return false;
    		}
    		first = first.next;
    		if (second == first) {
    			return true;
    		}
    	}
    	return false;
    }
}