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.

 package Algorithms.list;

 import Algorithms.algorithm.others.ListNode;

 /**
  * Definition for singly-linked list.
  * class ListNode {
  *     int val;
  *     ListNode next;
  *     ListNode(int x) {
  *         val = x;
  *         next = null;
  *     }
  * }
  */
 public class HasCycle {
     public boolean hasCycle(ListNode head) {
         if (head == null) {
             return false;
         }

         ListNode slow = head;
         ListNode fast = head;

         while (fast != null && fast.next != null) {
             slow = slow.next;
             fast = fast.next.next;
             if (slow == fast) {
                 return true;
             }
         }

         return false;
     }
 }