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"

  Constraints:

1 <= a.length, b.length <= 10⁴
a and b consist only of '0' or '1' characters.
Each string does not contain leading zeros except for the zero itself.