# -*- coding: utf-8 -*-

# !/usr/bin/env python

# Time: 2018/8/21 16:44

# Author: sty

# File: tree_operate_python.py

import sys

class TreeNode:

def __init__(self, x):

self.val = x

self.left = None

self.right = None

class Solution:

def preorderTraversal(self, root):

"""

:type root: TreeNode

:rtype: List[int]

"""

ret = []

stack = [root]

while stack:

node = stack.pop()

if node:

ret.append(node.val)

stack.append(node.right)

stack.append(node.left)

return ret

def list_to_treenode(input_values):

if not input_values:

return None

root = TreeNode(int(input_values[0]))

node_queue = [root]

front = 0

index = 1

while index < len(input_values):

node = node_queue[front]

front += 1

item = input_values[index]

index += 1

if item != "null":

left_num = int(item)

node.left = TreeNode(left_num)

node_queue.append(node.left)

if index >= len(input_values):

break

item = input_values[index]

index += 1

if item != "null":

right_num = int(item)

node.right = TreeNode(right_num)

node_queue.append(node.right)

return root

def main():

line = sys.stdin.readline().strip()

values = list(map(str, line.split()))

root = list_to_treenode(values)

pret = Solution().preorderTraversal(root)

print(pret)

if __name__ == '__main__':

main()

"""

input:

3 9 20 null null 15 7

output:

[3, 9, 20, 15, 7]

"""