9、環形鏈表

使用快慢指針，在環形鏈表內部，快指針遲早會追上慢指針。

bool hasCycle(struct ListNode *head)
{
    if(head == NULL || head->next == NULL)
        return false;
    struct ListNode *fast = head->next;		//fast = head將不能進入循環
    struct ListNode *slow = head;
    while(fast != head){
        if(fast == NULL || fast->next == NULL)//判斷爲非環形鏈表
            return false;
        fast = fast->next->next;
        slow = slow->next;
    }
    return true;
}