LeetCode: Sum Root to Leaf Numbers

class Solution {
public:
void dfs(TreeNode *p,int &sum,int curNum){
if(p->left == NULL && p ->right == NULL){
curNum *= 10;
curNum += p->val;
sum += curNum;
return;
}
if(p->left)
dfs(p->left,sum,curNum*10+p->val);
if(p->right)
dfs(p->right,sum,curNum*10+p->val);
}
int sumNumbers(TreeNode *root) {
if(root){
int ret = 0;
dfs(root,ret,0);
return ret;
}
return 0;
}
};