leetcode Linked List Cycle

判断链表中有没有环。

一个指针跑一次，一个指针跑两次，相遇就是有环

/**
 * 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 || !head -> next) return false;
        ListNode *l1 = head, *l2 = head -> next;
        while(l1 && l2)
        {
            if (l1 == l2)
                return true;
            l1 = l1 -> next; 
            l2 = l2 -> next;
            if (!l2)
                break;
            l2 = l2 -> next;
        }
        return false;
    }
};