【Linked List Cycle】cpp

/**
 * 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) {
            std::map<ListNode *, bool> ifNodeOccured;
            ListNode *p = head;
            while ( p )
            {
                if ( ifNodeOccured.find(p) != ifNodeOccured.end() ) return true;
                ifNodeOccured.insert(std::pair<ListNode *, bool>(p,true));
                p = p->next;
            }
            return false;
    }
};

/**
 * 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) return false;
            ListNode *p1 = head;
            ListNode *p2 = head;
            while ( p2->next && p2->next->next )
            {
                p2 = p2->next->next;
                p1 = p1->next;
                if (p2==p1) return true;
            }
            return false;
    }
};