sum-root-to-leaf-numbers

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