Longest K Unique Characters Substring

The problem. Given a string s of lowercase letters and a number k, return the length of the longest substring with exactly k distinct characters. If no substring has exactly k, return −1.

In aabacbebebe with k = 3, the longest substring with exactly 3 different characters is cbebebe, from index 4 to 10, length 7. It uses c, b and e.

Example

s = "aabacbebebe",  k = 3  ->  7     ("cbebebe")
s = "aaaa",         k = 2  ->  -1    (only one kind of letter)
s = "aabbcc",       k = 2  ->  4     ("aabb" or "bbcc")

Free account

Keep reading. It's free.

Sign up to read the rest of this lesson: 6 more sections, 2 drawings, a dry-run simulator and code in JavaScript, Python, Java and C++.

Still to come

1. Brute force: count the kinds in every substring
2. Optimal: grow from every start, stop early
3. Best solution: a sliding window with a count map
Comparing the approaches
Edge cases to test
Variations

Prev Next

Mark completed

Was this helpful?

ArraysContiguous, indexable memory — the foundation almost every other data structure builds on.12 lessons · 16 problems
StringsCharacter arrays with their own set of classic patterns — two pointers, sliding window, and more.19 lessons · 19 problems
QueueFirst-in, first-out — the structure behind task scheduling and breadth-first traversal.4 lessons · 5 problems

Longest K Unique Characters Substring

Example

Keep reading. It's free.

Keep exploring