create csharp/0145-binary-tree-postorder-traversal.cs

Author: NotADucc

csharp/0145-binary-tree-postorder-traversal.cs

@@ -0,0 +1,20 @@
+public class Solution {
+    public IList<int> PostorderTraversal(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;
+        }
+
+        Traverse(output, root.left);
+        Traverse(output, root.right);
+        output.Add(root.val);
+    }
+}