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"
      

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.