LeetCode: Linked List Cycle 解题报告

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?

SOLUTION 1:

经典快慢指针问题。如果存在环，fast, slow必然会相遇。就像2个速度不一样的人在环形跑道赛跑，总有一个时间他们会相遇。

如果fast到达了null，就是没有环，可以返回false.

 1 package Algorithms.list;
 2
 3 import Algorithms.algorithm.others.ListNode;
 4
 5 /**
 6  * Definition for singly-linked list.
 7  * class ListNode {
 8  *     int val;
 9  *     ListNode next;
10  *     ListNode(int x) {
11  *         val = x;
12  *         next = null;
13  *     }
14  * }
15  */
16 public class HasCycle {
17     public boolean hasCycle(ListNode head) {
18         if (head == null) {
19             return false;
20         }
21
22         ListNode slow = head;
23         ListNode fast = head;
24
25         while (fast != null && fast.next != null) {
26             slow = slow.next;
27             fast = fast.next.next;
28             if (slow == fast) {
29                 return true;
30             }
31         }
32
33         return false;
34     }
35 }