题目

You have seen many generic-type collections like Lists. When you want to use a List, you can create it for any type like List of int, List of string, List of arrays or …
Your task is easy; you have to create a customized generic class named Loop; (it is a closed loop like a circle to rotate left and right)
It has specific functionalities:

• You can add objects to it. It adds the object to the end of the Loop. The loop can also use collection initializer. (like a List)
• You can rotate it to the Right or Left. So the end and beginning of the loop are changed. I refer the test case to see more details.
• You can ShowFirst or PopOut the first item. Show just return the first item, while PopOut returns the first item while removing it from the loop.
• There is no situation to face empty list. You don’t need to handle any exception.

解题思路

此题需要完成的功能：添加、左右旋转（类似左轮手枪或保险柜锁那种）以及对首个元素的处理
1. 实现添加功能，需要继承System.Collections.IEnumerable，同时写上public System.Collections.IEnumerator GetEnumerator(){ return null; }，目的是为了使其可以初始化添加元素（就是数组大括号里面写上元素那种）；同时这里的添加指的是往最后一个元素后添加，因此要写成list.AddAfter（end，item）；
2. 此题名为Loop，意思即为这个类实例出来的列表是一个首尾相连的数组，里面的元素左移或右移，意味着对开头和结尾的元素影响最大，需要考虑这两者的变化；
3. 此题应该先考虑类的返回类型，这里设定为T，随后考虑存储元素的容器，这里使用的是linkedList（链表）和LinkedListNode（链表节点），这两个类中的属性用法需要注意，基本都会用到；
4. 需要考虑到这个链表一开始为空的情况，此时添加进去的元素既是start又是end;
5. 链表节点存储了该节点的元素value，以及该元素的下一个元素的指针；

代码

using System.Collections.Generic;

public class Loop<T>: System.Collections.IEnumerable 
    // ,System.Collections.Generic.IEnumerable<T>
    {
        private LinkedList<T> list;
        private LinkedListNode<T> start;
        private LinkedListNode<T> end;

        public Loop(){
            list = new LinkedList<T>();
        }

        public void Add(T item)
        {
            if(end == null){
                this.list.AddLast(item);
                this.start = list.First;
                this.end = list.Last;
            }
            else{
                this.list.AddAfter(end, item);
                this.end = this.end.Next;
            }
        }

        public void Rigth()
        {
            if(this.start.Previous != null){
                this.start = this.start.Previous;
            }
            else{
                this.start = this.list.Last;
            }

            if(this.end.Previous != null){
                this.end = this.end.Previous;
            }
            else {
                this.end = this.list.Last;
            }
        }

        public void Left()
        {
            if(this.start.Next != null){
                this.start = this.start.Next;
            }
            else{
                this.start = this.list.First;
            }

            if(this.end.Next != null){
                this.end = this.end.Next;
            }
            else {
                this.end = this.list.First;
            }
        }

        public T PopOut()
        {
            var val = this.start.Value;
            var oldStart = this.start;

            if(this.start.Next != null){
                this.start = this.start.Next;
            }
            else{
                this.start = this.list.First;
            }
            this.list.Remove(oldStart);

            return val;
        }

        public T ShowFirst()
        {
            return this.start.Value;
        }

        public System.Collections.IEnumerator GetEnumerator(){ return null; }
    }

参考资料

1. linkedList（链表）https://docs.microsoft.com/zh-cn/dotnet/api/system.collections.generic.linkedlist-1?view=netframework-4.7.2
2. LinkedListNode（链表节点）https://docs.microsoft.com/zh-cn/dotnet/api/system.collections.generic.linkedlistnode-1?view=netframework-4.7.2
3. IEnumerable Interface（接口）
   https://docs.microsoft.com/zh-cn/dotnet/api/system.collections.ienumerable?view=netframework-4.7.2