387\. First Unique Character in a String

Easy

Given a string `s`, _find the first non-repeating character in it and return its index_. If it does not exist, return `-1`.

**Example 1:**

**Input:** s = "leetcode"

**Output:** 0

**Example 2:**

**Input:** s = "loveleetcode"

**Output:** 2

**Example 3:**

**Input:** s = "aabb"

**Output:** -1

**Constraints:**

*   <code>1 <= s.length <= 10<sup>5</sup></code>
*   `s` consists of only lowercase English letters.