求二叉树最大层数

int maxDepth(TreeNode* root) {
    if (root == nullptr) {
        return 0;
    }
    
    constexpr int MIN_DEPTH    = 0;
    constexpr int initLayerNum = 1;
    
    function<int(TreeNode*&, int)> check;
    check = [&](TreeNode*& node, int num)
    {
        if (node == nullptr) {
            return MIN_DEPTH;
        }
        
        if (node->left == nullptr && node->right == nullptr){
            return num;
        }
        else{
            return max(check(node->left, num + 1), check(node->right, num + 1));
        }
    };
    
    return check(root, initLayerNum);
}