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

class Solution {
public:
  int subarraysWithKDistinct(vector<int>& A, int K) {
    return atMostK(A, K) - atMostK(A,K-1);
  }
  int atMostK(vector<int>& A, int K) {
    int i = 0
 
,res = 0;
    unordered_map<int,int> mp;
    for(int j=0; j<A.size(); j++) {
      if(!mp[A[j]]++) K--;
      while(K<0){
        if(!--mp[A[i]]) K++;
        i++;
      }   
      res += j - i + 1;
    }   
    return res;
  }
};

LC 992. Subarrays with K Different Integers