LeetCode 141. Linked List Cycle 判断链表是否有环 C++/Java

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

To represent a cycle in the given linked list, we use an integer poswhich represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.

Example 2:

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.

Example 3:

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

判断链表是否有环：使用快慢指针去判断，如果有环，fast快指针先进入环，slow慢指针后进入，经过一定步的操作，快慢指针在环上相遇。
方法一：（C++）

 class Solution {
 public:
     bool hasCycle(ListNode *head) {
         ListNode *fast=head,*slow=head;
         while(fast&&fast->next){
             slow=slow->next;
             fast=fast->next->next;
             if(slow==fast)
                 return true;
         }
         return false;
     }
 };

方法二：（Java）

 public class Solution {
     public boolean hasCycle(ListNode head) {
         Set<ListNode> s=new HashSet();
         while(head!=null){
             if(s.contains(head))
                 return true;
             else
                 s.add(head);
             head=head.next;
         }
         return false;
     }
 }

Java集合中含有contains（）方法，判断其是否已经含有此节点，如果没有，则使用add()添加进去