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0992 - Subarrays with K Different Integers (Hard)

https://leetcode.com/problems/subarrays-with-k-different-integers/

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 * 10^4
  • 1 <= nums[i], k <= nums.length

Approach 1: Sliding Window​

Written by @wingkwong
class Solution {
public:
    int atMost(vector<int>& nums, int k) {
        // standard sliding window pattern
        int n = nums.size(), i = 0, ans = 0;
        unordered_map<int, int> m;
        for (int j = 0; j < n; j++) {
            // step 1: make the condition invalid
            k -= !m[nums[j]]++;
            // step 2: if the condition is failed, 
            // it means we need to shrink the window
            // by adding k to make the condition valid
            while(k < 0) k += !(--m[nums[i++]]);
            // step 3: add the distance to ans
            ans += j - i + 1;
        }
        return ans;
    }
    
    int subarraysWithKDistinct(vector<int>& nums, int k) {
        // exactky k = at most k - at most (k - 1)
        return atMost(nums, k) - atMost(nums, k - 1);
    }
};