LeetCode——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?

原题链接：https://oj.leetcode.com/problems/linked-list-cycle/

题目：给定一个链表。推断它是否有环。

继续：

你能不用额外的空间解决吗？

思路：使用两个指针fast,slow，fast每次向前走两步，slow每次向前走一步。假设二者相应的节点相等。即存在环。

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

// Definition for singly-linked list.
class ListNode {
int val;
ListNode next;

ListNode(int x) {
val = x;
next = null;
}
}