Add Binary
<p>Given two binary strings <code>a</code> and <code>b</code>, return <em>their sum as a binary string</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> a = "11", b = "1"
<strong>Output:</strong> "100"
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> a = "1010", b = "1011"
<strong>Output:</strong> "10101"
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= a.length, b.length &lt;= 10<sup>4</sup></code></li>
	<li><code>a</code> and <code>b</code> consist&nbsp;only of <code>&#39;0&#39;</code> or <code>&#39;1&#39;</code> characters.</li>
	<li>Each string does not contain leading zeros except for the zero itself.</li>
</ul>
Bit Manipulation; Math; String; Simulation
func addBinary(a string, b string) string {
	i, j := len(a)-1, len(b)-1
	ans := []byte{}
	for carry := 0; i >= 0 || j >= 0 || carry > 0; i, j = i-1, j-1 {
		if i >= 0 {
			carry += int(a[i] - '0')
		}
		if j >= 0 {
			carry += int(b[j] - '0')
		}
		ans = append(ans, byte(carry%2+'0'))
		carry /= 2
	}
	for i, j := 0, len(ans)-1; i < j; i, j = i+1, j-1 {
		ans[i], ans[j] = ans[j], ans[i]
	}
	return string(ans)
}