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Problem Statement:

Given an integer array nums and an integer k, return the number of good subarrays of nums.

A good array is an array where the number of different integers in that array is exactly k.

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

A subarray is a contiguous part of an array.

 

Example 1:

Input: nums = [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: nums = [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].

 

Constraints:

  • 1 <= nums.length <= 2 * 104
  • 1 <= nums[i], k <= nums.length

Solution:

We solve the problem of subarray with at most K distinct integers and use it for K and K-1 to get the answer.

 
class Solution {
public:
    int subarraysWithKDistinct(vector<int>& nums, int k)
    {
        return atMostK(nums,k)-atMostK(nums,k-1);
    }
    int atMostK(vector<int>& nums, int k) 
    {
        int n = nums.size(), i=0, res=0;
        unordered_map<int,int> H;
        for (int j=0; j<n; j++)
        {
            H[nums[j]]++;
            while(H.size()>k)
            {
                H[nums[i]]--;
                if (H[nums[i]]==0) H.erase(nums[i]);
                i++;
            }
            res += (j-i+1);
        }
        return res;
    }
};