Subarrays with K Different Integers

Description

Given an array A of positive integers, call a (contiguous, not necessarily distinct) subarray of A good if the number of different integers in that subarray is exactly K.

(For example, [1,2,3,1,2] has 3 different integers: 1, 2, and 3.)

Return the number of good subarrays of A.

 

Example 1:

Input: A = [1,2,1,2,3], K = 2
Output: 7
Explanation: Subarrays formed with exactly 2 different integers: [1,2], [2,1], [1,2], [2,3], [1,2,1], [2,1,2], [1,2,1,2].

Example 2:

Input: A = [1,2,1,3,4], K = 3
Output: 3
Explanation: Subarrays formed with exactly 3 different integers: [1,2,1,3], [2,1,3], [1,3,4].

 

Note:

1. 1 <= A.length <= 20000
2. 1 <= A[i] <= A.length
3. 1 <= K <= A.length

Solution(javascript)

/**
 * @param {number[]} A
 * @param {number} K
 * @return {number}
 */
const subarraysWithKDistinct = function (A, K) {
  const atMost = (count) => {
    const map = {}
    let left = 0
    let distinct = 0
    let result = 0
    for (let right = 0; right < A.length; right++) {
      const num = A[right]
      map[num] = (map[num] || 0) + 1

      if (map[num] === 1) {
        distinct += 1
      }
      while (distinct > count) {
        map[A[left]] -= 1
        if (map[A[left]] === 0) {
          distinct -= 1
        }

        left += 1
      }
      result += right - left + 1
    }
    return result
  }
  return atMost(K) - atMost(K - 1)
}