# 思路

使用 recursive 遍歷判斷 tree 的深度

# 參考程式碼

	/**
	* Definition for a binary tree node.
	* struct TreeNode {
	* int val;
	* TreeNode *left;
	* TreeNode *right;
	* TreeNode() : val(0), left(nullptr), right(nullptr) {}
	* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
	* TreeNode(int x, TreeNode left, TreeNode right) : val(x), left(left), right(right) {}
	* };
	*/

	static auto fast_io = []
	{
	ios::sync_with_stdio(false);
	cout.tie(nullptr);
	cin.tie(nullptr);
	return 0;
	}();

	class Solution {
	public:
	int maxDepth(TreeNode* root)
	{
	if (!root) return 0;
	return (1 + max(maxDepth(root->left), maxDepth(root->right)));
	}
	};