LeetCode: Linked List Cycle

快慢指针法检查单链表是否存在环，注意判断无环的条件。

/**
 * 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* dummy = new ListNode(0);
        dummy->next = head;
        ListNode* p1 = dummy;
        ListNode* p2 = dummy;
        while (p2!=NULL && p2->next!=NULL) {
            p2 = p2->next->next;
            p1 = p1->next;
            if (p2 == p1) return true;
        }
        return false;
    }
};