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.