144_Binary_Tree_Preorder_Traversal.py

"""
144. Binary Tree Preorder Traversal

Easy
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]
Explanation:

Example 2:
Input: root = [1,2,3,4,5,null,8,null,null,6,7,9]
Output: [1,2,4,5,6,7,3,8,9]
Explanation:

Example 3:
Input: root = []
Output: []

Example 4:
Input: root = [1]
Output: [1]

Constraints:

The number of nodes in the tree is in the range [0, 100].
-100 <= Node.val <= 100

"""

# Definition for a binary tree node.
class TreeNode(object):
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right
class Solution(object):
    def preorderTraversal(self, root):
        """
        :type root: Optional[TreeNode]
        :rtype: List[int]
        """

        if root is None:
            return[]

        result=[]
        stack=[root]

        while stack:
            current = stack.pop()
            result.append(current.val)

            if current.right:
                stack.append(current.right)
            if current.left:
                stack.append(current.left)

        return result