https://leetcode.com/problems/add-binary/
Given two binary strings a and b, return their sum as a binary string.
Example 1:
Input: a = "11", b = "1"
Output: "100"
Example 2:
Input: a = "1010", b = "1011"
Output: "10101"
1 <= a.length, b.length <= 10^4
a and b consist only of '0' or '1' characters.
Each string does not contain leading zeros except for the zero itself.