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 public TreeNode addOneRow(TreeNode root, int v, int d) {
        if (d < 2) {
            TreeNode newroot = new TreeNode(v);
            if (d == 0) newroot.right = root;
            else newroot.left = root;
            return newroot;
        }
        if (root == null) return null;
        root.left = addOneRow(root.left, v, d == 2 ? 1 : d-1);
        root.right = addOneRow(root.right, v, d == 2 ? 0 : d-1);
        return root;
    }

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private void dfs(TreeNode root, int v, int level, int d) {
        if (root == null) return;
        if (level < d - 1) {
            dfs(root.left, v, level + 1, d);
            dfs(root.right, v, level + 1, d);
        } else {
            TreeNode t = root.left;
            root.left = new TreeNode(v);
            root.left.left = t;
            t = root.right;
            root.right = new TreeNode(v);
            root.right.right = t;
        }
        
    }
    public TreeNode addOneRow(TreeNode root, int v, int d) {
        if (d == 1) {
            TreeNode newRoot = new TreeNode(v);
            newRoot.left = root;
            return newRoot;
        }
        dfs(root, v, 1, d);
        return root;
    }
}

public TreeNode addOneRow(TreeNode root, int v, int d) {
        if (d == 1) {
            TreeNode newroot = new TreeNode(v);
            newroot.left = root;
            return newroot;
        }
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        for (int i = 0; i < d-2; i++) {
            int size = queue.size();
            for (int j = 0; j < size; j++) {
                TreeNode t = queue.poll();
                if (t.left != null) queue.add(t.left);
                if (t.right != null) queue.add(t.right);
            }
        }
        while (!queue.isEmpty()) {
            TreeNode t = queue.poll();
            TreeNode tmp = t.left;
            t.left = new TreeNode(v);
            t.left.left = tmp;
            tmp = t.right;
            t.right = new TreeNode(v);
            t.right.right = tmp;
        }
        return root;
    }