LeetCode : 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.

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        long long total = 0;
        long long val = 0;
        sum(root, val, total);
        return total;
    }
    void sum(TreeNode * root, long long  val, long long & total){
        if(!root) {
            return;
        }
        if(!root->left && !root->right){
            total += val * 10 + root->val;
            return;
        }
        int curValue = val * 10 + root->val;
        sum(root->left, curValue, total);
        sum(root->right, curValue, total);
    }
};