144-Binary-Tree-Preorder-Traversal.py

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

# 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]:
        # One liner - recursive        
        # return [root.val] + self.preorderTraversal(root.left) + self.preorderTraversal(root.right) if root else []
    
        # Iteration Method
        if root is None:
            return []
        stack, output = [root, ], []
        
        while stack:
            root = stack.pop()
            if root is not None:
                output.append(root.val)
                if root.right is not None:
                    stack.append(root.right)
                if root.left is not None:
                    stack.append(root.left)
        return output