LeetCode 141. Linked List Cycle （链表循环）

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

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

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题目标签：Linked List

　　题目给了我们一个 Linked List，让我们判断它是否循环。

　　利用快，慢指针，快指针一次走2步，慢指针一次走1步，如果循环，快慢指针一定会相遇。

Java Solution:

Runtime beats 98.15%

完成日期：06/09/2017

关键词：singly-linked list；cycle

关键点：fast, slow pointers

 /**
  * 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)
             return false;

         ListNode slow = head;
         ListNode fast = head;

         do
         {
             if(fast.next == null || fast.next.next == null) // if fast reaches to end, it doens't have a cycle
                 return false;

             fast = fast.next.next; // fast moves 2 steps
             slow = slow.next; // slow moves 1 step

         } while(fast != slow);

         return true; // if fast catches slow, meaning it has a cycle
     }
 }

参考资料：N/A

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