Sum Root to Leaf Numbers [LeetCode]

Problem description: http://oj.leetcode.com/problems/sum-root-to-leaf-numbers/

Basic idea: To store the num vector in every node of tree by starting from leaf, the go up util to root.

 class Solution {
 public:
     vector<vector<int>> subNumbers(TreeNode *root) {
         vector<vector<int>> sums;
         if(root == NULL)
             return sums;

         if(root->left == NULL && root->right == NULL){
             vector<int> seq;
             seq.push_back(root->val);
             sums.push_back(seq);
             return sums;
         }

         vector<vector<int>> left_sums = subNumbers(root -> left);
         for(auto item: left_sums) {
             item.insert(item.begin(), root->val);
             sums.push_back(item);
         }

         vector<vector<int>> right_sums = subNumbers(root -> right);
         for(auto item: right_sums) {
             item.insert(item.begin(), root->val);
             sums.push_back(item);
         }
         return sums;
     }

     int pow10(int n) {
         int ret = ;
         for(int i = ; i < n; i++)
             ret = ret * ;

         return ret;
     }

     int sumNumbers(TreeNode *root) {
         // Note: The Solution object is instantiated only once and is reused by each test case.
         int sum = ;
         vector<vector<int>> sums = subNumbers(root);
         for(auto v : sums){
             int tmp_sum = ;
             for(int i = v.size() - ; i >= ; i -- ) {
                 tmp_sum += v[i] * pow10(v.size() -  - i);
             }
             sum += tmp_sum;
         }
         return sum;
     }
 };