Add two binary numbers given as strings and return the result as a binary string.
Example 1
Input: a = "1010", b = "1011"
Output: "10101"
Explanation: 1010 + 1011 = 10101.
Output: "10101"
Explanation: 1010 + 1011 = 10101.
Example 2
Input: a = "11", b = "1"
Output: "100"
Explanation: 11 + 1 = 100 in binary.
Output: "100"
Explanation: 11 + 1 = 100 in binary.