added isinstance check

Author: akshaysharma096

traverals/binary_tree_traversals.py

@@ -40,31 +40,35 @@ def build_tree():
 
 
 def pre_order(node):
-    if not node:
+    if not isinstance(node, TreeNode) or not node:
+        print("Invalid input")
         return
     print(node.data, end=" ")
     pre_order(node.left)
     pre_order(node.right)
 
 
 def in_order(node):
-    if not node:
+     if not isinstance(node, TreeNode) or not node:
+        print("Invalid input")
         return
     in_order(node.left)
     print(node.data, end=" ")
     in_order(node.right)
 
 
 def post_order(node):
-    if not node:
+     if not isinstance(node, TreeNode) or not node:
+        print("Invalid input")
         return
     post_order(node.left)
     post_order(node.right)
     print(node.data, end=" ")
 
 
 def level_order(node):
-    if not node:
+     if not isinstance(node, TreeNode) or not node:
+        print("Invalid input")
         return
     q = queue.Queue()
     q.put(node)