107. Binary Tree Level Order Traversal II - cocoder39/coco39_LC GitHub Wiki

class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        vector<vector<int>> res;
        helper(res, root, 0);
        reverse(res.begin(), res.end());
        return res;
    }
private:
    void helper(vector<vector<int>>& res, TreeNode* node, int level) {
        if(! node) {
            return;
        }

        if (res.size() == level) { // a start of a new level
            res.push_back(vector<int>());
        }
        res[level].push_back(node->val);
        helper(res, node->left, level + 1);
        helper(res, node->right, level + 1);
    }
};