根据所给树的头结点创建分层数组

vector<vector<int>> levelOrder(TreeNode* root) {
    vector<vector<int>> levelOrderTraversal;
    
    function<void(TreeNode*, size_t)> traversal;
    traversal = [&levelOrderTraversal, &traversal](TreeNode* node, size_t levelNo){
        if (node == nullptr){
            return;
        }
        
        if(levelNo >= levelOrderTraversal.size()){
            levelOrderTraversal.push_back(vector<int>());
        }
        
        levelOrderTraversal[levelNo].push_back(node->val);
        ++levelNo;
        traversal(node->left,  levelNo);
        traversal(node->right, levelNo);
    };
    traversal(root, 0);
    return levelOrderTraversal;
}