Add One Row to Tree
<p>Given the <code>root</code> of a binary tree and two integers <code>val</code> and <code>depth</code>, add a row of nodes with value <code>val</code> at the given depth <code>depth</code>.</p>

<p>Note that the <code>root</code> node is at depth <code>1</code>.</p>

<p>The adding rule is:</p>

<ul>
	<li>Given the integer <code>depth</code>, for each not null tree node <code>cur</code> at the depth <code>depth - 1</code>, create two tree nodes with value <code>val</code> as <code>cur</code>&#39;s left subtree root and right subtree root.</li>
	<li><code>cur</code>&#39;s original left subtree should be the left subtree of the new left subtree root.</li>
	<li><code>cur</code>&#39;s original right subtree should be the right subtree of the new right subtree root.</li>
	<li>If <code>depth == 1</code> that means there is no depth <code>depth - 1</code> at all, then create a tree node with value <code>val</code> as the new root of the whole original tree, and the original tree is the new root&#39;s left subtree.</li>
</ul>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0600-0699/0623.Add%20One%20Row%20to%20Tree/images/addrow-tree.jpg" style="width: 500px; height: 231px;" />
<pre>
<strong>Input:</strong> root = [4,2,6,3,1,5], val = 1, depth = 2
<strong>Output:</strong> [4,1,1,2,null,null,6,3,1,5]
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0600-0699/0623.Add%20One%20Row%20to%20Tree/images/add2-tree.jpg" style="width: 500px; height: 277px;" />
<pre>
<strong>Input:</strong> root = [4,2,null,3,1], val = 1, depth = 3
<strong>Output:</strong> [4,2,null,1,1,3,null,null,1]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 10<sup>4</sup>]</code>.</li>
	<li>The depth of the tree is in the range <code>[1, 10<sup>4</sup>]</code>.</li>
	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
	<li><code>-10<sup>5</sup> &lt;= val &lt;= 10<sup>5</sup></code></li>
	<li><code>1 &lt;= depth &lt;= the depth of tree + 1</code></li>
</ul>
Tree; Depth-First Search; Breadth-First Search; Binary Tree
/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func addOneRow(root *TreeNode, val int, depth int) *TreeNode {
	if depth == 1 {
		return &TreeNode{Val: val, Left: root}
	}
	var dfs func(root *TreeNode, d int)
	dfs = func(root *TreeNode, d int) {
		if root == nil {
			return
		}
		if d == depth-1 {
			l, r := &TreeNode{Val: val, Left: root.Left}, &TreeNode{Val: val, Right: root.Right}
			root.Left, root.Right = l, r
			return
		}
		dfs(root.Left, d+1)
		dfs(root.Right, d+1)
	}
	dfs(root, 1)
	return root
}