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 ;
};