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#144. Binary Tree Preorder Traversal | LeetCode Python Solution

Problem​

LeetCode Problem

  1. Binary Tree Preorder Traversal

Python Solution without Recursion​

# 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:
    # Preorder (Root, Left, Right)
    def preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        # Return an empty list if there is no tree
        if root is None:
            return []

        # Output list
        output = []

        # Push root node to stack
        stack = [root]
        while len(stack) > 0:
            # Pop from stack
            curr = stack.pop()

            # Add the current node's value to the output list
            output.append(curr.val)

            # Push right node to stack
            if curr.right is not None:
                stack.append(curr.right)

            # Push left node to stack
            if curr.left is not None:
                stack.append(curr.left)

        return output