107.binary-tree-level-order-trav

class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
                if(!root) return {};
        // deque
        deque<TreeNode*> q;
        vector<vector<int>> ans;
        q.push_back(root);
        int deep = 0;

        while(!q.empty()){
            int size = q.size();
            // init 
            ans.push_back({});
            while(size--) {
                TreeNode* n = q.front();q.pop_front();
                ans[deep].push_back(n->val);
                // none
                if(n->left) q.push_back(n->left);
                if(n->right) q.push_back(n->right);
            }
            deep++;
        }
        reverse(ans.begin(),ans.end());    
        return ans;
    }
};