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?

思路：

快慢指针。两个指针同时从head出发，一个每次加一，一个每次加二，如果相遇的话就存在环。

代码

    bool hasCycle(ListNode *head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(head == NULL)
            return false;
        if(head->next == NULL)
            return false;
        ListNode *slow = head, *fast = head;
        while(true){
            if(fast && fast->next){
                fast = fast->next->next;
            }
            else{
                return false;
            }
            slow = slow->next;
            if(fast == slow)
                return true;
        }
    }