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:

1 <= s.length <= 105
s consists of only lowercase English letters.