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==nullptr)return false;
        ListNode* fast = head;
        ListNode* slow = head;
        while(fast && fast->next){
            fast = fast->next->next;
            slow = slow->next;
            if(slow==fast){
                slow = head;
                while(slow != fast){
                    slow = slow->next;
                    fast = fast->next;
                }
                return true;
            }
        }
        return false;
    }
};