Longest substring without repeating characters

Challenge

Given a C-string s of ASCII bytes, return the length of the longest substring with all distinct characters. Implement int longest_unique(const char *s).

Why this matters

The sliding-window pattern with a frequency map is one of the highest-yield algorithmic techniques: it appears in network rate-limiters, log analyzers, and DNA sequence search.

Input format

A null-terminated ASCII string (bytes 0..127).

Output format

The length as int.

Constraints

O(n) time. No nested loops.

Starter code

int longest_unique(const char *s) { /* TODO */ return 0; }

Common mistakes

Recomputing the start index incorrectly (must use max(start, last_seen+1)); using a hash map when a 128-entry array suffices for ASCII; off-by-one on the length calculation.

Edge cases to handle

Empty string returns 0. All identical characters return 1. All distinct returns strlen.

Complexity

O(n) time, O(128) memory.

Background lessons

• Hashing — turning keys into table indices