LC102 Binary Tree Level Order Traversal

class Solution {
public:
vector<vector<int> > levelOrder(TreeNode *root) {
vector<vector<int>> result;
traverse(root, 1, result);
return result;
}
void traverse(TreeNode *root, size_t level, vector<vector<int>> &result) {
if (!root) return;
if (level > result.size())
result.push_back(vector<int>());
result[level-1].push_back(root->val);
traverse(root->left, level+1, result);
traverse(root->right, level+1, result);
}
};