257. Binary Tree Paths

Easy

Given the root of a binary tree, return all root-to-leaf paths in any order.

A leaf is a node with no children.

class Solution:
    def binaryTreePaths(self, root: Optional[TreeNode]) -> List[str]:
        if root is None:
            return []

        return self.helper(root, [])

    def helper(self, root, paths):
        if len(paths) == 0:
            new_paths = [str(root.val)]
        else:
            new_paths = [p + f"->{root.val}" for p in paths]

        if root.left is None and root.right is None:
            return new_paths

        left = []
        if root.left is not None:
            left = self.helper(root.left, new_paths)

        right = []
        if root.right is not None:
            right = self.helper(root.right, new_paths)

        return left + right


class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right