Find first non-repeating character index

You are given two separate coding tasks.

Task 1: First Unique Character

Given a string s (lowercase/ASCII letters), return the index of the first character that appears exactly once in the string. If no such character exists, return -1.

Input: s (string)

Output: integer index (0-based) or -1

Examples

• s = "leetcode" → 0 (character 'l' )
• s = "loveleetcode" → 2 (character 'v' )
• s = "aabb" → -1

Task 2: Highest Average Score

You are given a list of student score records. Each record contains a student_id (string) and a score (integer). A student can appear multiple times.

Compute the average score per student (mean of all that student’s scores), and return the maximum average across all students.

Input: list of pairs [(student_id, score), ...]

Output: the highest average (as a floating-point value or as an exact rational; specify your choice during the interview)

Example Input: [("a", 80), ("b", 90), ("a", 100)]

• avg(a) = (80 + 100) / 2 = 90
• avg(b) = 90 Output: 90

Notes / Clarifications to ask:

• Should ties return just the value, or also the student id(s)?
• How should rounding be handled if the average is not an integer?