Given a string s, return 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.