LeetCode 129. 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:
int sum;

void process(TreeNode* root, int& sum, int cur){

cur = cur * 10 + root->val;
if(!root->left && !root->right){
sum += cur;
return;
}
if(root->left) process(root->left, sum, cur);
if(root->right) process(root->right, sum, cur);

}

int sumNumbers(TreeNode* root) {
if(!root) return 0;

sum = 0;
process(root, sum, 0);
return sum;
}
};