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 = 1;
        for(int i = 0; i < n; i++)
            ret = ret * 10;
            
        return ret;
    }
    
    int sumNumbers(TreeNode *root) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        int sum = 0;
        vector<vector<int>> sums = subNumbers(root);
        for(auto v : sums){
            int tmp_sum = 0;
            for(int i = v.size() - 1; i >= 0; i -- ) {
                tmp_sum += v[i] * pow10(v.size() - 1 - i);
            }
            sum += tmp_sum;
        }
        return sum;
    }
};