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) {}
 * };
 * 快慢指针
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if(!head){
            return false;
        }
        ListNode* slow = head;
        ListNode* fast = head;
        while (fast != NULL && slow != NULL){
            slow = slow->next;
            fast = fast->next;
            if(fast){
                fast = fast->next;
            }
            else{
                return false;
            }

            if(slow==fast){
                return true;
            }
        }
        return false;
    }
};