Merge branch 'master' of https://github.com/haptear/hello-algo

# Conflicts:
#	codes/csharp/chapter_array_and_linkedlist/list.cs
#	codes/csharp/chapter_array_and_linkedlist/my_list.cs
#	codes/csharp/include/PrintUtil.cs
#	codes/csharp/include/TreeNode.cs

codes/csharp/chapter_array_and_linkedlist/list.cs

@@ -45,7 +45,7 @@ namespace hello_algo.chapter_array_and_linkedlist
             Console.WriteLine("在索引 3 处插入数字 6 ，得到 list = " + string.Join(",", list));
 
             /* 删除元素 */
-            list.Remove(3);
+            list.RemoveAt(3);
             Console.WriteLine("删除索引 3 处的元素，得到 list = " + string.Join(",", list));
 
             /* 通过索引遍历列表 */

codes/csharp/include/ListNode.cs

@@ -10,7 +10,7 @@ namespace hello_algo.include
     public class ListNode
     {
         public int val;
-        public ListNode next;
+        public ListNode? next;
 
         /// <summary>
         /// Generate a linked list with an array
@@ -26,7 +26,7 @@ namespace hello_algo.include
         /// </summary>
         /// <param name="arr"></param>
         /// <returns></returns>
-        public static ListNode ArrToLinkedList(int[] arr)
+        public static ListNode? ArrToLinkedList(int[] arr)
         {
             ListNode dum = new ListNode(0);
             ListNode head = dum;
@@ -44,7 +44,7 @@ namespace hello_algo.include
         /// <param name="head"></param>
         /// <param name="val"></param>
         /// <returns></returns>
-        public static ListNode GetListNode(ListNode head, int val)
+        public static ListNode? GetListNode(ListNode? head, int val)
         {
             while (head != null && head.val != val)
             {