Linked List Cycle

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?

这道题要用双指针，但我想试一下投机取巧的办法行不行，果然A了

/**
 * 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(null == head)
            return false;
        ListNode point = head;
        while(null != point){
            if(point.val != Integer.MIN_VALUE)
                point.val = Integer.MIN_VALUE;
            else
                return true;
                point = point.next;
        }        
        return false;
    }
}

 双指针，一个走一步一个走两步，有环两个一定会相遇

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