sum root to leaf numbers

int ans = 0;
public int (TreeNode root) {
    if (root == null)
        return 0;
    dfs(root, 0);
    return ans;
}

public void dfs(TreeNode root, int sum) {
    if (root == null)
        return;
    int val = root.val;
    if (root.left == null && root.right == null) {
        ans += (sum * 10 + val);
        return;
    }
    dfs(root.left, sum * 10 + val);
    dfs(root.right, sum * 10 + val);
}