<p>Given a string <code>s</code>, <em>find the first non-repeating character in it and return its index</em>. If it does not exist, return <code>-1</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> s = "leetcode"
<strong>Output:</strong> 0
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> s = "loveleetcode"
<strong>Output:</strong> 2
</pre><p><strong class="example">Example 3:</strong></p>
<pre><strong>Input:</strong> s = "aabb"
<strong>Output:</strong> -1
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 10<sup>5</sup></code></li>
	<li><code>s</code> consists of only lowercase English letters.</li>
</ul>