@@ -13,37 +13,37 @@ def __init__(self, data):
 
 
 def build_tree():
+print("\n********Press N to stop entering at any point of time********\n")
 print("Enter the value of the root node: ", end="")
-data = eval(input())
-if data < 0:
-return None
-else:
-q = queue.Queue()
-tree_node = TreeNode(data)
-q.put(tree_node)
-while not q.empty():
-node_found = q.get()
-print("Enter the left node of %s: " % node_found.data, end="")
-left_data = eval(input())
-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:
-return tree_node
-elif right_data >= 0:
-right_node = TreeNode(right_data)
-node_found.right = right_node
-q.put(right_node)
+check=input()
+if check=='N' or check=='n':
+return None
+data=int(check)
+q = queue.Queue()
+tree_node = TreeNode(data)
+q.put(tree_node)
+while not q.empty():
+node_found = q.get()
+print("Enter the left node of %s: " % node_found.data, end="")
+check=input()
+if check=='N' or check =='n':
+return tree_node
+left_data = int(check)
+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="")
+check = input()
+if check == 'N' or check == 'n':
+return tree_node
+right_data = int(check)
+right_node = TreeNode(right_data)
+node_found.right = right_node
+q.put(right_node)
 
 
 def pre_order(node):
 if not isinstance(node, TreeNode) or not node:
-#print("Invalid input")
 return
 print(node.data, end=" ")
 pre_order(node.left)