Basic example
Input
arr = [1, 2, 1, 2, 3], K = 2
Output
4
The longest subarray with at most 2 distinct numbers is [1,2,1,2] with length 4.
Full lesson preview
Find the longest subarray with at most K distinct elements
Find the length of the longest contiguous subarray that contains at most K distinct elements using a sliding window and frequency map.
Python practice18 minTwo Pointers & Sliding WindowIntermediateLast updated March 26, 2026
Problem statement
Given an array (or list) arr and an integer K, return the length of the longest contiguous subarray that contains at most K distinct elements. If K is 0, return 0.
Task
Apply the variable-size sliding window technique with a frequency map to track distinct counts and maximize window length.
Examples
Input format
A list arr (elements can be any hashable type) and an integer K.
Output format
An integer representing the maximum length.
Constraints
0 <= K <= len(arr) <= 10^5. Elements are hashable (e.g., ints, strings). Time O(n).
Samples
Sample 1
Input
arr = ['a', 'b', 'a', 'b', 'c'], K = 2
Output
4
Longest window ['a','b','a','b'] length 4.