Write first_unique(s) that returns the first character of s that appears exactly once in the whole string. Return "" if every character repeats or the string is empty. Comparison is case-sensitive, and every character counts (spaces and punctuation included).
Examples
Input: s = "swiss"
Output: "w"
Input: s = "aabb"
Output: ""
Input: s = "programming"
Output: "p"
Constraints
0 <= len(s) <= 10**5, printable ASCII- Target: O(n) time
Goals
- Count frequencies in one pass
- Make a second pass in original order
- Return a sentinel when nothing qualifies