/**
<p>Given a binary tree, find its maximum depth.</p>

<p>The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.</p>

<p><strong>Note:</strong>&nbsp;A leaf is a node with no children.</p>

<p><strong>Example:</strong></p>

<p>Given binary tree <code>[3,9,20,null,null,15,7]</code>,</p>

<pre>
    3
   / \
  9  20
    /  \
   15   7</pre>

<p>return its depth = 3.</p>
<p>给定一个二叉树，找出其最大深度。</p>

<p>二叉树的深度为根节点到最远叶子节点的最长路径上的节点数。</p>

<p><strong>说明:</strong>&nbsp;叶子节点是指没有子节点的节点。</p>

<p><strong>示例：</strong><br>
给定二叉树 <code>[3,9,20,null,null,15,7]</code>，</p>

<pre>    3
   / \
  9  20
    /  \
   15   7</pre>

<p>返回它的最大深度&nbsp;3 。</p>
<p>给定一个二叉树，找出其最大深度。</p>

<p>二叉树的深度为根节点到最远叶子节点的最长路径上的节点数。</p>

<p><strong>说明:</strong>&nbsp;叶子节点是指没有子节点的节点。</p>

<p><strong>示例：</strong><br>
给定二叉树 <code>[3,9,20,null,null,15,7]</code>，</p>

<pre>    3
   / \
  9  20
    /  \
   15   7</pre>

<p>返回它的最大深度&nbsp;3 。</p>
**/


/**
 * Definition for a binary tree node.
 * class TreeNode(var `val`: Int = 0) {
 *     var left: TreeNode? = null
 *     var right: TreeNode? = null
 * }
 */
class Solution {
    fun maxDepth(root: TreeNode?): Int {
        
    }
}