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;
        for( ; fast != nullptr && fast -> next !=  nullptr; ){
            fast = fast -> next -> next;
            slow = slow -> next;
            if(fast == slow){
                return true;
            }
        }
        return false;
    }
};

用快慢指针，循环用while和for都可以