Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

一個+1 一個+2 看看能不能相遇。註意代碼不要寫亂了...以後盡量用p1 p2 這種變量名，不要用p ,q讓人看著眼花繚亂

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

141. Linked List Cycle