Sum Root to Leaf Numbers

/**
 * 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 {
public:
    string _covert_to_string(int i) {
        char c[8];
        snprintf(c,7, "%d", i);
        return static_cast<string>(c);
    }
    void get_sum_of_tree(TreeNode* root, string branch_value, int &sum) {
        if (NULL == root) {
            return;
        } else if (root->left == NULL && root->right == NULL) {
            branch_value = branch_value + _covert_to_string(root->val);
            sum += atoi(branch_value.c_str());
            return;
        }
        branch_value = branch_value + _covert_to_string(root->val);
        get_sum_of_tree(root->left, branch_value, sum);
        get_sum_of_tree(root->right, branch_value, sum);
    }
    
    int sumNumbers(TreeNode* root) {
        string b = "";
        int sum = 0;
        get_sum_of_tree(root, b, sum);
        return sum;
    }
};