Binary Tree Level Order Traversal II

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
private:
    int maxDep;
    vector<vector<int>> res;
public:
    void traversal(int dep, TreeNode *root) {

        if(root == NULL)
            return;

        if(res.size() < dep + 1) {
            vector<int> tmp;
            tmp.push_back(root->val);
            res.push_back(tmp);
        } else {
            res[dep].push_back(root->val);
        }
        traversal(dep + 1, root->left);
        traversal(dep + 1, root->right);

    }
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        traversal(0, root);
        return vector<vector<int>>(res.rbegin(), res.rend());

    }
};