Longest Window with K Distinct

Implement static int longestWindow(int[] xs, int k) — the length of the longest contiguous run containing at most k distinct values. k <= 0 → 0. Use the sliding-window pattern with a frequency map. [1,2,1,2,3], k=2 → 4 (the run [1,2,1,2]). [1,1,1], k=1 → 3. Empty → 0.

Difficulty: intermediate