Longest substring without repeating characters

Challenge

Find the length of the longest run of distinct characters in a string.

Task

Given a C-string s of ASCII bytes, implement int longest_unique(const char *s) that returns the length of the longest contiguous substring in which every character is distinct.

Input

s: a NUL-terminated ASCII string (bytes 0..127), possibly empty.

Output

int: the length of the longest substring with no repeated characters.

Example

"abcabcbb"   ->   3    ("abc")
"bbbbb"      ->   1    ("b")
"pwwkew"     ->   3    ("wke")
""           ->   0

Edge cases

• Empty string returns 0.
• All identical characters return 1; an all-distinct string returns its length.

Rules

• Solve in O(n) with a sliding window and a last-seen index table (no nested loops).

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

s: NUL-terminated ASCII string (bytes 0..127); may be empty.

Output format

int: length of the longest substring with all distinct characters.

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