[Leetcode] Add Binary

Problem:

Given two binary strings, return their sum (also a binary string).
For example, a = "11" b = "1",  return "100".

Similar to Decimal Addition, using a carry to store the current digit and carry to next digit.
For dth digit
   carry = carry + a[d] + b[d]//if a or b's length < d, just add zero
   c[d] = carry%base
   carry = carry/base
End

If using a string builder, the result is in reverse order.
Note finally the carry may be non zero.

Code: