Return the index of the first unique character in the string.
Example 1
Input: s = "leetcode"
Output: 0
Explanation:
Output: 0
Explanation:
Example 2
Input: s = "aabb"
Output: -1
Explanation: There are no unique characters, so -1 is returned.
Output: -1
Explanation: There are no unique characters, so -1 is returned.