Given a integer represented as a string, we need to get the sum of all possible substrings of this string.
Input : num = “1234”
Output : 1670
Sum = 1 + 2 + 3 + 4 + 12 + 23 +
34 + 123 + 234 + 1234
= 1670
Input : num = “421”
Output : 491
Sum = 4 + 2 + 1 + 42 + 21 + 421 = 491