LeetCode 141. Linked List Cycle

原題連接在這裏：https://leetcode.com/problems/linked-list-cycle/

題目：

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

Follow up:
Can you solve it without using extra space?

題解：

快慢指針，一快runner, 每次走兩步. 一慢walker, 每次走一步，如果它倆相遇就是有cycle.

Time Complexity: O(n). Space: O(1).

AC Java:

 1 /**
 2  * Definition for singly-linked list.
 3  * class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public boolean hasCycle(ListNode head) {
14         if(head == null || head.next == null){
15             return false;
16         }
17         ListNode walker = head;
18         ListNode runner = head;
19         while(runner.next != null && runner.next.next != null){
20             walker = walker.next;
21             runner = runner.next.next;
22             if(walker == runner){
23                 return true;
24             }
25         }
26         return false;
27     }
28 }

 跟上Linked List Cycle II, Circular Array Loop.