leetcode 137: Linked List Cycle

最新推荐文章于 2025-05-07 22:36:04 发布

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本文介绍了一种高效判断链表中是否存在循环的方法，并提供了一个不使用额外空间的具体实现方案。通过快慢指针技巧，该算法能在O(n)时间内完成检测。

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Linked List Cycle

Total Accepted: 3575 Total Submissions: 9771

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

Follow up:
Can you solve it without using extra space?

/**
 * 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.
        if(head==NULL || head->next==NULL) return false;
        ListNode* p = head->next;
        while(p!=NULL && p->next!=NULL) {
            if(p==head || p->next==head) return true;
            head = head->next;
            p = p->next->next;
        }
        return false;
    }
};