9、环形链表

使用快慢指针，在环形链表内部，快指针迟早会追上慢指针。

bool hasCycle(struct ListNode *head)
{
    if(head == NULL || head->next == NULL)
        return false;
    struct ListNode *fast = head->next;		//fast = head将不能进入循环
    struct ListNode *slow = head;
    while(fast != head){
        if(fast == NULL || fast->next == NULL)//判断为非环形链表
            return false;
        fast = fast->next->next;
        slow = slow->next;
    }
    return true;
}