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.