LeetCode OJ：Linked List Cycle（链表循环）

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

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

判断一个链表是否存在环，维护快慢指针就可以，如果有环那么快指针一定会追上慢指针，代码如下：

class Solution {
public:
    bool hasCycle(ListNode *head) {
        ListNode * slow, * fast;
        slow = fast = head;
        while(slow && fast){
            slow = slow->next;
            fast = fast->next;
            if(fast) fast = fast->next;
            if(fast && slow && slow == fast)
                return true;
        }
        return false;
    }
};