Problem URL: Binary tree preorder traversal

We will traverse the tree with recursive DFS, as it is preorder traversal, we first append the root, then the left subtree and then right subtree.

# 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]:
        res = []

        def traverse(root, res):
            if not root:
                return
            res.append(root.val)
            traverse(root.left, res)
            traverse(root.right, res)

        traverse(root, res)
        return res

Time Complexity: O(n)
Space Complexity: O(n)