Merge pull request #3654 from NotADucc/0144

create 0144-binary-tree-preorder-traversal.cs

Author: Ykhan799

Diff for: csharp/0144-binary-tree-preorder-traversal.cs

@@ -0,0 +1,20 @@
+public class Solution {
+    public IList<int> PreorderTraversal(TreeNode root)
+    {
+        var output = new List<int>();
+        Traverse(output, root);
+        return output;
+    }
+
+    private void Traverse(List<int> output, TreeNode root) 
+    {
+        if (root is null)
+        {
+            return;
+        }
+
+        output.Add(root.val);
+        Traverse(output, root.left);
+        Traverse(output, root.right);
+    }
+}