leetcode 141. 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?

题意：

      判断一个链表是否有环。

解决方案：

      双指针，快指针每次走两步，慢指针每次走一步，

            如果有环，快指针和慢指针会在环内相遇，fast == slow，这时候返回true。

            如果没有环，返回false.

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