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) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (!head || !head->next) return false;
        ListNode* fast = head;
        ListNode* slow = head;

        do 
        {
            fast = fast->next;
            if (!fast) return false;
            fast = fast->next;
            if (!fast) return false;
            slow = slow->next;
            if (!slow)
            return false;
        }
        while (fast != slow);
        return true;
        
    }
};