LeetCode(141)(142) Linked List Cycle I II

[code]/**
 * 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) {

        set<ListNode *> visited;
        ListNode *p1;

        p1 = head;
        while(p1 != NULL) {

            if(visited.count(p1))

                return true;

            else

                visited.insert(p1);

            p1 = p1->next;

        }

        return false;

    }
};

[code]/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {

        set<ListNode *> visited;
        ListNode *p1;

        p1 = head;
        while(p1 != NULL) {

            if(visited.count(p1)) 

                return p1;

            else 

                visited.insert(p1);

            p1 = p1->next;

        }

        return NULL;

    }
};