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:
void solve(TreeNode *root)
{
if(root -> left == NULL && root ->right == NULL){
sum += root->val;
return;
}
if(root->left){
root->left->val += root->val * 10;
solve(root->left);
}

if(root->right){
root->right->val += root->val * 10;
solve(root->right);
}
}
int sumNumbers(TreeNode *root) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
sum = 0;
if(root == NULL) return sum;
solve(root);
return sum;
}
int sum ;
};