67\. Add Binary

Easy

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:**

*   <code>1 <= a.length, b.length <= 10<sup>4</sup></code>
*   `a` and `b` consist only of `'0'` or `'1'` characters.
*   Each string does not contain leading zeros except for the zero itself.