Binary Tree Preorder Traversal

144. Binary Tree Preorder Traversal

Difficulty: Easy

Related Topics: Stack, Tree, DFS, Binary Tree

Given the root of a binary tree, return the preorder traversal of its nodes’ values.

Example 1:

Input: root = [1,null,2,3] Output: [1,2,3]

Example 2:

Input: root = [] Output: []

Example 3:

Input: root = [1] Output: [1]

Code:

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        values = []
        stack = [root]
        while stack:
            node = stack.pop()
            if node:
                values.append(node.val)
                stack.append(node.right)
                stack.append(node.left)
        return values

Feedbacks:

I found the values of elements in binary tree by iteratively. Make a stack to store a root. As root exists, I pop it and store it to values list, and append node.right and node.left to stack, and re-do the same steps continuously untill I get to null values.