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) {
        ListNode* fast = head;
        ListNode* slow = head;

        while(fast != NULL && fast->next != NULL){  // 如果有环，快慢指针一定会相遇
            fast = fast->next->next;
            slow = slow->next;

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

        return false;  // 出循环说明存在NULL, 环不会有NULL
    }
};