private final static BigInteger MOD = BigInteger.valueOf(1000000007);

    /*
     * Input: 123456 
     * Role of '2': 2, 20, 200, 2000, 20000, 200000
     */
    public static int substrings(String number) {
        int length = number.length();
        BigInteger totalSum = BigInteger.valueOf(0);
        BigInteger multiplier = BigInteger.valueOf(1);
        BigInteger sum = BigInteger.valueOf(0);

        // Iterate over each digit from the end to the start
        for (int i = length - 1; i >= 0; i--) {
            BigInteger digit = BigInteger.valueOf(Character.getNumericValue(number.charAt(i)));
            sum = (sum.add(digit.multiply(multiplier))).mod(MOD);
            totalSum = (totalSum.add(sum)).mod(MOD);
            multiplier = (multiplier.multiply(BigInteger.valueOf(10)).add(BigInteger.valueOf(1))).mod(MOD);
        }

        return totalSum.intValue();
    }