First Unique Character in a String

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

Examples

Example 1:

Input: s = "leetcode"

Output: 0

Explanation: The character 'l' at index 0 is the first character that does not repeat.

Example 2:

Input: s = "loveleetcode"

Output: 2

Explanation: The character 'v' at index 2 is the first non-repeating character.

Example 3:

Input: s = "aabb"

Output: -1

Explanation: Every character in the string repeats, so there is no unique character.

Constraints

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