0090_P992_Subarrays_with_K_Different_Integers.cpp

/* 
Day: 90
Problem Number: 992 (https://leetcode.com/problems/subarrays-with-k-different-integers)
Date: 30-03-2024
Description:
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 * 10^4
* 1 <= nums[i], k <= nums.length

Code:  */
class Solution {
public:
    int subarraysWithKDistinct(vector<int>& nums, int k) {
        vector<int> left = f(nums, k);
        vector<int> right = f(nums, k - 1);
        int ans = 0;
        for (int i = 0; i < nums.size(); ++i) {
            ans += right[i] - left[i];
        }
        return ans;
    }

    vector<int> f(vector<int>& nums, int k) {
        int n = nums.size();
        vector<int> pos(n);
        int cnt[n + 1];
        memset(cnt, 0, sizeof(cnt));
        int s = 0;
        for (int i = 0, j = 0; i < n; ++i) {
            if (++cnt[nums[i]] == 1) {
                ++s;
            }
            for (; s > k; ++j) {
                if (--cnt[nums[j]] == 0) {
                    --s;
                }
            }
            pos[i] = j;
        }
        return pos;
    }
};