leetcode环形链表（c++创建循环链表）

[LeetCode] 141. Linked List Cycle 单链表中的环

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

To represent a cycle in the given linked list, we use an integerposwhich represents the position (0-indexed)in the linked list where tail connects to. Ifposis-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.

Follow up:

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

这道题是快慢指针的经典应用。只需要设两个指针，一个每次走一步的慢指针和一个每次走两步的快指针，如果链表里有环的话，两个指针最终肯定会相遇。实在是太巧妙了，要是我肯定想不出来。代码如下：

C++ 解法：

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

Java 解法：

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

Github 同步地址：

https://github.com/grandyang/leetcode/issues/141

类似题目：

Linked List Cycle II

Happy Number

参考资料：

https://leetcode.com/problems/linked-list-cycle/

https://leetcode.com/problems/linked-list-cycle/discuss/44489/O(1)-Space-Solution

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原文链接：https://www.cnblogs.com/grandyang/p/4137187.html