LeetCode 141. Linked List Cycle

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

题意：给你一个链表，找出是否它的内部有环形的圆。

思路：来两个指针，一个一次走一步，一个一次走两步，如果第二个能追上第一个就代表有圆。

java代码：

/**
 * 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)  
            return false;  
        ListNode slow = head;  
        ListNode fast = head;  
        while (fast.next != null && fast.next.next != null) {  
            slow = slow.next;  
            fast = fast.next.next;  
            if (slow == fast)  
                return true;  
        }  
        return false;  
    }  
}