141. Linked List Cycle*

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

题目描述

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.

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 using $O(1)$ (i.e. constant) memory?

C++ 实现 1

使用快慢指针来判断是否有环. 第一种写法是初始化时, 快慢指针不指向同一个节点.

另外, 142. Linked List Cycle II** 这题要求找到环的起始节点.

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

C++ 实现 2

这种写法是快慢指针初始时指向相同的节点.

class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (!head || !head->next)
            return false;

        ListNode *slow = head, *fast = head;
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
            if (fast == slow)
                return true;
        }
        return false;
    }
};