（Java）leetcode-141 Linked List Cycle

港控/mmm° 2022-02-28 02:28 198阅读 0赞

# 题目 #

【判断链表是否有环】

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 pos which 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.  
![在这里插入图片描述][circularlinkedlist.png]

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.  
![在这里插入图片描述][circularlinkedlist_test2.png]

Example 3:

Input: head = \[1\], pos = -1  
Output: false  
Explanation: There is no cycle in the linked list.  
![在这里插入图片描述][circularlinkedlist_test3.png]

Follow up:

Can you solve it using O(1) (i.e. constant) memory?

# 思路 #

设置一快一慢两个指针，快的一次走两步，慢的一次走一步，若链表中存在环，则两个指针必定会相遇，否则不存在环  
时间复杂度 O(n）  
空间复杂度 O(1)

## 代码1 ##

*/
    public class Solution { 
        public boolean hasCycle(ListNode head) { 
            if(head == null) return false;
            ListNode fast = head;
            ListNode slow = head;
            while(  fast != null && fast.next != null ){ 
            	fast = fast.next.next;
            	slow = slow.next;
            	if(fast == slow) return true;
    
            }
            return false;
    
        }
    }

## 提交结果 ##

Runtime: 0 ms, faster than 100.00% of Java online submissions for Linked List Cycle.  
Memory Usage: 38.7 MB, less than 29.76% of Java online submissions for Linked List Cycle.

## 代码2 ##

答案的解法，思路一样

public boolean hasCycle(ListNode head) { 
        if (head == null || head.next == null) { 
            return false;
        }
        ListNode slow = head;
        ListNode fast = head.next;
        while (slow != fast) { 
            if (fast == null || fast.next == null) { 
                return false;
            }
            slow = slow.next;
            fast = fast.next.next;
        }
        return true;
    }

## 提交结果 ##

Runtime: 0 ms, faster than 100.00% of Java online submissions for Linked List Cycle.  
Memory Usage: 38.5 MB, less than 54.54% of Java online submissions for Linked List Cycle.

[circularlinkedlist.png]: https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png
[circularlinkedlist_test2.png]: https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png
[circularlinkedlist_test3.png]: https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png