Solution2.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode addOneRow(TreeNode root, int val, int depth) {
        if (depth == 1) {
            return new TreeNode(val, root, null);
        }
        Deque<TreeNode> q = new ArrayDeque<>();
        q.offer(root);
        int i = 0;
        while (!q.isEmpty()) {
            ++i;
            for (int k = q.size(); k > 0; --k) {
                TreeNode node = q.pollFirst();
                if (node.left != null) {
                    q.offer(node.left);
                }
                if (node.right != null) {
                    q.offer(node.right);
                }
                if (i == depth - 1) {
                    node.left = new TreeNode(val, node.left, null);
                    node.right = new TreeNode(val, null, node.right);
                }
            }
        }
        return root;
    }
}