623. Add One Row to Tree

https://leetcode.com/problems/add-one-row-to-tree/

Python

Edge case很多，要多思考一下
- depth == 1
- depth == max_tree_depth

class Solution:
    def addOneRow(self, root: Optional[TreeNode], val: int, depth: int) -> Optional[TreeNode]:
        if depth == 1:
            return TreeNode(val=val, left=root)

        def dfs(node, level, direction):
            if not node:
                return None if level != depth else TreeNode(val=val)

            if level != depth:
                node.left = dfs(node.left, level+1, 'L')
                node.right = dfs(node.right, level+1, 'R')
                return node

            return TreeNode(
                val=val,
                left=dfs(node, level+2, 'L') if direction == 'L' else None,
                right=dfs(node, level+2, 'R') if direction == 'R' else None
            )

        root.left = dfs(root.left, 2, 'L')
        root.right = dfs(root.right, 2, 'R')
        return root

Python​

Python