Code
#include <bits/stdc++.h>
using namespace std;

class Solution {
public:
    // Helper function to count subarrays with at most K distinct integers
    int atMostK(vector<int>& nums, int K) {
        unordered_map<int, int> freq;
        int left = 0, count = 0;

        // Traverse the array with right pointer
        for (int right = 0; right < nums.size(); right++) {
            // If it's a new unique element, decrease K
            if (freq[nums[right]] == 0) {
                K--;
            }

            // Increment frequency of current element
            freq[nums[right]]++;

            // Shrink the window if distinct count > K
            while (K < 0) {
                freq[nums[left]]--;
                if (freq[nums[left]] == 0) {
                    K++;
                }
                left++;
            }

            // Count all subarrays ending at current right
            count += (right - left + 1);
        }

        return count;
    }

    // Main function to return number of subarrays with exactly K distinct integers
    int subarraysWithKDistinct(vector<int>& nums, int k) {
        return atMostK(nums, k) - atMostK(nums, k - 1);
    }
};

// Driver code
int main() {
    Solution sol;
    vector<int> nums = {1, 2, 1, 2, 3};
    int k = 2;
    cout << sol.subarraysWithKDistinct(nums, k) << endl;
    return 0;
}