feat(csharp/backtracking): add csharp code in backtracking (#463)

codes/csharp/chapter_backtracking/preorder_find_nodes.cs

@@ -0,0 +1,51 @@
+/**
+ * File: preorder_find_nodes.cs
+ * Created Time: 2023-04-17
+ * Author: hpstory (hpstory1024@163.com)
+ */
+
+using hello_algo.include;
+using NUnit.Framework;
+
+namespace hello_algo.chapter_backtracking;
+
+public class preorder_find_nodes
+{
+    static List<TreeNode> res;
+
+    /* 前序遍历 */
+    static void preOrder(TreeNode root)
+    {
+        if (root == null)
+        {
+            return;
+        }
+        if (root.val == 7)
+        {
+            // 记录解
+            res.Add(root);
+        }
+        preOrder(root.left);
+        preOrder(root.right);
+    }
+
+    [Test]
+    public void Test()
+    {
+        TreeNode root = TreeNode.ArrToTree(new int?[] { 1, 7, 3, 4, 5, 6, 7 });
+        Console.WriteLine("\n初始化二叉树");
+        PrintUtil.PrintTree(root);
+
+        // 前序遍历
+        res = new List<TreeNode>();
+        preOrder(root);
+
+        Console.WriteLine("\n输出所有值为 7 的节点");
+        List<int> vals = new List<int>();
+        foreach (TreeNode node in res)
+        {
+            vals.Add(node.val);
+        }
+        Console.WriteLine(string.Join(" ", vals));
+    }
+}