First Unique Character
Given a string s, find the index of the first character that appears exactly once. If no such character exists, return -1.
Input
s: a string
Output
An integer - the index of the first unique character, or -1 if none exists
Examples
Example 1:
Input: s = "leetcode"
Output: 0
Explanation: The character 'l' at index 0 appears only once. It is the first unique character.
Example 2:
Input: s = "loveleetcode"
Output: 2
Explanation: Character frequencies: l(3), o(2), v(1), e(4), t(1), c(1), d(1). The first character that appears only once is 'v' at index 2.
Example 3:
Input: s = "aabb"
Output: -1
Explanation: All characters appear more than once.