@@ -1,7 +1,7 @@
 """
 This is pure python implementation of tree traversal algorithms
 """
-
+from __future__ import print_function
 import queue
 
 
@@ -25,22 +25,25 @@ def build_tree():
 node_found = q.get()
 print("Enter the left node of %s: " % node_found.data, end="")
 left_data = eval(input())
-if left_data >= 0:
+if left_data < 0:
+return tree_node
+elif left_data >= 0:
 left_node = TreeNode(left_data)
 node_found.left = left_node
 q.put(left_node)
 print("Enter the right node of %s: " % node_found.data, end="")
 right_data = eval(input())
-if right_data >= 0:
+if right_data < 0:
+return tree_node
+elif right_data >= 0:
 right_node = TreeNode(right_data)
 node_found.right = right_node
 q.put(right_node)
-return tree_node
 
 
 def pre_order(node):
 if not isinstance(node, TreeNode) or not node:
-print("Invalid input")
+#print("Invalid input")
 return
 print(node.data, end=" ")
 pre_order(node.left)