Subarrays with K Different Integers

export function subarraysWithKDistinct(nums: number[], k: number): number {
  const n = nums.length;
  const counts = new Array(n + 1).fill(0);
 
  let answer = 0;
  let [start, mid] = [0, 0];
  let remain = k;
  for (const num of nums) {
    counts[num] += 1;
    if (counts[num] === 1) {
      remain -= 1;
      if (remain < 0) {
        counts[nums[mid]] = 0;
        mid += 1;
        start = mid;
      }
    }
 
    if (remain <= 0) {
      while (1 < counts[nums[mid]]) {
        counts[nums[mid]] -= 1;
        mid += 1;
      }
      answer += mid - start + 1;
    }
  }
 
  return answer;
}