Sum of Substrings

Samantha and Sam are playing a numbers game. Given a number as a string, no leading zeros, determine the sum of all integer values of substrings of the string. For example, if the string is 42, the substrings are 4, 2 and 42. Their sum is 48. Given an integer as a string, sum all of its substrings cast as integers. As the number may become large, return the value modulo 10^9+7. ###Input: - A si