141. Linked List Cycle

Problem:

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

Follow up:

Can you solve it without using extra space?

Analysis:

利用快慢双指针，如果有环则快指针会追上慢指针，否则快指针先到达链表尾。

Answer:

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