Sum Root to Leaf Numbers

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.
An example is the root-to-leaf path 1->2->3 which represents the number 123.
Find the total sum of all root-to-leaf numbers.
For example,
1
/
2 3
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Return the sum = 12 + 13 = 25.

Solution: 1. Recursion (add to sum when reaching the leaf).
2. Iterative solution.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int sumNumbers(TreeNode *root) {
        int sum = 0;
        sumNumberRe(root, 0, sum);
        return sum;
    }
    
    void sumNumberRe(TreeNode* root, int num, int &sum)
    {
        if(!root) return;
        num = num * 10 + root->val;
        if(!root->left && !root->right) {
            sum += num;
        }
        sumNumberRe(root->left, num, sum);
        sumNumberRe(root->right, num, sum);
    }
};