Solution.ts

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */

function addOneRow(root: TreeNode | null, val: number, depth: number): TreeNode | null {
    function dfs(root, d) {
        if (!root) {
            return;
        }
        if (d == depth - 1) {
            root.left = new TreeNode(val, root.left, null);
            root.right = new TreeNode(val, null, root.right);
            return;
        }
        dfs(root.left, d + 1);
        dfs(root.right, d + 1);
    }
    if (depth == 1) {
        return new TreeNode(val, root);
    }
    dfs(root, 1);
    return root;
}