Sum Root to Leaf Numbers

/**
 * 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 sum = 0;
    void dfs(TreeNode *root,int num){
        if(!root) return ;//空数
        if(!root->left&&!root->right){//到达叶子节点
            sum+=num*10+root->val;
            return ;
        }
        dfs(root->left,num*10+root->val);//遍历左右子树
        dfs(root->right,num*10+root->val);
        return ;
    }
    int sumNumbers(TreeNode *root) {
        dfs(root,0);
        return sum;
    }
};