I use insert to add in the front of the vector.

and use res.size()-level to find where the node should be in the res.

but insert really takes a lot of time.

In fact, it takes 70ms.

```
vector<vector<int>> res;
vector<int> tmp;
void helper(TreeNode *curr,int level){
if(curr==NULL)return;
int s=res.size();
if(level==s){
res.insert(res.begin(),tmp);
s++;
}
level++;
res[s-level].push_back(curr->val);
helper(curr->left,level);
helper(curr->right,level);
}
vector<vector<int> > levelOrderBottom(TreeNode *root) {
helper(root,0);
return res;
}
```