141. Linked List Cycle

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本文探讨了如何判断链表中是否存在循环。通过一个简单的算法，我们可以在一定条件下有效地检测到链表中的循环。文章提供了三个示例，展示了不同情况下链表循环的检测结果。

141. Linked List Cycle

Easy

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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?

Accepted

493,678

Submissions

1,271,790

/**
 * 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) {
        if(head==NULL) return false;
        int cnt=0;int flag=0;
        while(head->next!=NULL){
            head=head->next;
            cnt++;
            if(cnt>10000){//循环大于10000的话肯定有环
                flag=1;break;
            }
        }
        return flag;
    }
};