Leetcode: Linked List Cycle

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

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

分析：如果一个linked list中有环，那么在用一个快指针和一个慢指针遍历该linked list时，这两个指针必会相交。代码如下：

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