[LeetCode] 144. Binary Tree Preorder Traversal

Problem : https://leetcode.com/problems/binary-tree-preorder-traversal/

Recursive solution:

# 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: TreeNode) -> List[int]:
        def preorder(node):
            if node:
                yield node.val
                yield from preorder(node.left)
                yield from preorder(node.right)

        return list(preorder(root))

Iterative solution:

# 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: TreeNode) -> List[int]:
        result = []
        stack = [root] if root else []
        
        while stack:
            node = stack.pop()
            
            result.append(node.val)
            
            for child in filter(None, [node.right, node.left]):
                stack.append(child)
        
        return result

Edited on 04/20/2021. Update iterative solution. Same approach can apply to N-array tree too.

Edited on 04/20/2021. Update recursive solution. Use 'yield' and 'yield from'.