# ---
# title: 144. Binary Tree Preorder Traversal
# id: problem144
# author: Tian Jun
# date: 2020-10-31
# difficulty: Medium
# categories: Stack, Tree
# link: <https://leetcode.com/problems/binary-tree-preorder-traversal/description/>
# hidden: true
# ---
# 
# Given the `root` of a binary tree, return _the preorder traversal of its nodes
# ' values_.
# 
# 
# 
# **Example 1:**
# 
# ![](https://assets.leetcode.com/uploads/2020/09/15/inorder_1.jpg)
# 
#     
#     
#     Input: root = [1,null,2,3]
#     Output: [1,2,3]
#     
# 
# **Example 2:**
# 
#     
#     
#     Input: root = []
#     Output: []
#     
# 
# **Example 3:**
# 
#     
#     
#     Input: root = [1]
#     Output: [1]
#     
# 
# **Example 4:**
# 
# ![](https://assets.leetcode.com/uploads/2020/09/15/inorder_5.jpg)
# 
#     
#     
#     Input: root = [1,2]
#     Output: [1,2]
#     
# 
# **Example 5:**
# 
# ![](https://assets.leetcode.com/uploads/2020/09/15/inorder_4.jpg)
# 
#     
#     
#     Input: root = [1,null,2]
#     Output: [1,2]
#     
# 
# 
# 
# **Constraints:**
# 
#   * The number of nodes in the tree is in the range `[0, 100]`.
#   * `-100 <= Node.val <= 100`
# 
# 
# 
# **Follow up:**
# 
# Recursive solution is trivial, could you do it iteratively?
# 
# 
# 
# 
## @lc code=start
using LeetCode

## add your code here:
## @lc code=end
