题解 | 判断链表中是否有环

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
#include <cstddef>
class Solution {
public:
    bool hasCycle(ListNode *head) { // 快慢指针，快指针每次两单位，慢指针一个单位
        if (head == NULL) {
            return false;
        }

        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;
    }
};