LeetCode - Linked List Cycle

链表循环检测

最新推荐文章于 2025-09-11 19:19:55 发布

原创最新推荐文章于 2025-09-11 19:19:55 发布 · 462 阅读

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文章标签：

#leetcode #linked list

本文介绍了一种使用快慢指针的方法来判断链表中是否存在循环的经典算法问题。通过两个速度不同的指针遍历链表，如果存在循环，则两指针最终会在循环内相遇。

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

http://oj.leetcode.com/problems/linked-list-cycle/

Solution:

Very popular problem. Use two pointer, p1 move two steps every time and p2 move one step every time. If there is a cycle, p1 and p2 will meet together in the cycle.

https://github.com/starcroce/leetcode/blob/master/linked_list_cycle.cpp

// 68 ms for 16 cases
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        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;
    }
};