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

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

題意：

給定一個鏈表，判斷是否有環

思路：

快慢指針

若有環，則快慢指針一定會在某個節點相遇(此處省略證明)

代碼：

 1 public class Solution {
 2     public boolean hasCycle(ListNode head) {
 3         ListNode fast = head;
 4         ListNode slow = head;
 
 5         while(fast != null && fast.next != null){
 6             fast = fast.next.next;
 7             slow = slow.next;
 8             if(fast == slow) return true;
 9         }
10         return false;
11     }
12 }

[leetcode]141. Linked List Cycle判斷鏈表是否有環