binaryTreePreorderTraversal.py

#! /usr/bin/env python3
# -*- coding: utf-8 -*-

# Source: https://leetcode.com/problems/binary-tree-preorder-traversal/
# Author: Miao Zhang
# Date:   2021-01-21

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        res = []
        if not root:
            return res
        res.append(root.val)
        res += self.preorderTraversal(root.left)
        res += self.preorderTraversal(root.right)
        
        return res

class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        if not root:
            return res
        stack = []
        while stack or root:
            if root:
                res.append(root.val)
                stack.append(root)
                root = root.left
            else:
                node = stack.pop()
                root = node.right
        return res