Pattern - At Most K

Problem: Count subarrays with at most $k$ distinct elements.

Approach: For each right endpoint, find smallest valid left. Count: $right - left + 1$ subarrays ending at right.

Trick: "Exactly $k$" = "At most $k$" $-$ "At most $k-1$". Counting exactly $k$ directly is complex; subtracting two simpler counts avoids edge cases.