Update 707.设计链表，添加C#版

problems/0707.设计链表.md

@@ -1485,6 +1485,77 @@ impl MyLinkedList {
 }
 ```
 
+### C#
+```C#
+class ListNode
+{
+    public int val;
+    public ListNode next;
+    public ListNode(int val) { this.val = val; }
+}
+public class MyLinkedList
+{
+    ListNode dummyHead;
+    int count;
+
+    public MyLinkedList()
+    {
+        dummyHead = new ListNode(0);
+        count = 0;
+    }
+
+    public int Get(int index)
+    {
+        if (index < 0 || count <= index) return -1;
+        ListNode current = dummyHead;
+        for (int i = 0; i <= index; i++)
+        {
+            current = current.next;
+        }
+        return current.val;
+    }
+
+    public void AddAtHead(int val)
+    {
+        AddAtIndex(0, val);
+    }
+
+    public void AddAtTail(int val)
+    {
+        AddAtIndex(count, val);
+    }
+
+    public void AddAtIndex(int index, int val)
+    {
+        if (index > count) return;
+        index = Math.Max(0, index);
+        count++;
+        ListNode tmp1 = dummyHead;
+        for (int i = 0; i < index; i++)
+        {
+            tmp1 = tmp1.next;
+        }
+        ListNode tmp2 = new ListNode(val);
+        tmp2.next = tmp1.next;
+        tmp1.next = tmp2;
+    }
+
+    public void DeleteAtIndex(int index)
+    {
+
+        if (index >= count || index < 0) return;
+        var tmp1 = dummyHead;
+        for (int i = 0; i < index; i++)
+        {
+            tmp1 = tmp1.next;
+        }
+        tmp1.next = tmp1.next.next;
+        count--;
+
+    }
+}
+```
+
 <p align="center">
 <a href="https://programmercarl.com/other/kstar.html" target="_blank">
   <img src="../pics/网站星球宣传海报.jpg" width="1000"/>