Example 1
Input
s = "araaci", k = 2
Output
4
Longest substring with exactly 2 distinct characters is "araa" (length 4).
Full lesson preview
Find the length of the longest substring with exactly K distinct characters
Given a string and an integer K, find the length of the longest substring that contains exactly K distinct characters.
Python practice18 minStrings – Sliding Window & FrequencyIntermediateLast updated March 25, 2026
Problem statement
Given a string s and an integer k, return the length of the longest substring of s that contains exactly k distinct characters. If no such substring exists, return 0.
Examples:
- s = "araaci", k = 2 -> the longest substring with exactly 2 distinct characters is "araa" of length 4.
- s = "cbbebi", k = 3 -> the longest is "cbbeb" or "bbebi" with length 5.
Task
Apply sliding window and frequency counting to maintain a window with exactly K distinct characters and maximize its length.
Examples
Input format
Two inputs: a string s and an integer k.
Output format
An integer representing the length of the longest substring that contains exactly k distinct characters (0 if none).
Constraints
- 0 <= len(s) <= 10^5
- 0 <= k <= len(s)
- Aim for O(n) time using a sliding window and hashmap.
Samples
Sample 1
Input
s = "cbbebi", k = 3
Output
5
Longest substrings with exactly 3 distinct characters include "cbbeb" and "bbebi" (length 5).