LeetCode: 141. Linked List Cycle 链表是否有环

最新推荐文章于 2025-05-07 22:36:04 发布

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最新推荐文章于 2025-05-07 22:36:04 发布

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本文介绍了一种使用O(1)内存复杂度检测链表中是否存在循环的方法。通过快慢指针技巧，当快指针与慢指针相遇时，表明链表存在循环。示例包括输入为[3,2,0,-4]且pos为1的情况，输出为true。

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试题：
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?
代码：

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head==null) return false;
        ListNode f = head, s = head.next;
        while(s!=null && s.next!=null){
            if(f==s) return true;
            f = f.next;
            s = s.next.next;
        }
        return false;
    }
}