114 判断一个链表是否存在环并返回环起点

方法一 活用set

   ListNode *detectCycle(ListNode *head) {
        set<ListNode *> save;
        while (head != nullptr)
            if (save.find(head) != save.end())
                return *save.find(head);
            else {
                save.emplace(head);
                head = head->next;
            }
        return nullptr;
    }

方法二 快慢指针

    ListNode *detectCycle(ListNode *head) {
        if(head==nullptr)
            return nullptr;
        ListNode *slow = head, *fast = head;
        while(true){
            if(fast->next==nullptr||fast->next->next==nullptr)
                return nullptr;
            slow=slow->next;
            fast=fast->next->next;
            if(slow==fast){
                while(head!=fast){
                    head=head->next;
                    fast=fast->next;
                }
                return head;
            }
        }
    }