class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        vector<vector<int>>ans;
        if(root==nullptr){
            return ans;
        }
        queue<TreeNode*>q;
        q.push(root);
        q.push(nullptr);
        vector<int>temp;
        while(!q.empty()){
            TreeNode*f=q.front();
            if(f==nullptr){
                ans.push_back(temp);
                q.pop();
                if(!q.empty()){
                    q.push(nullptr);
                }
                temp.clear();
            }
            else{
                temp.push_back(f->val);
                q.pop();
                if(f->left){
                    q.push(f->left);
                }
                if(f->right){
                    q.push(f->right);
                }
            }
        }
        reverse(ans.begin(),ans.end());
        return ans;
    }
};