Update bt_postorder.py

trees/bt_postorder.py

@@ -4,8 +4,34 @@
 
 def postorder(root) -> list:
         
-  if root is None:
+    if root is None:
       return []
     
-  return postorder(root.left) + postorder(root.right) + [root.val] 
+    return postorder(root.left) + postorder(root.right) + [root.val] 
+        
+
+def postorder_iterative(root) -> list:
+        
+    stack, result = [], []
+    node = root
+    
+    while node or stack:
+
+            while node:
+                if node.right:
+                    stack.append(node.right)
+                stack.append(node)
+                node = node.left
+            
+            node = stack.pop()
+            
+            if stack and node.right == stack[-1]:
+                stack[-1] = node
+                node = node.right
+
+            else:
+                result.append(node.val)
+                node = None
+                
+    return result