992. Subarrays with K Different Integers

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https://leetcode.com/problems/subarrays-with-k-different-integers/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 * 104
  • 1 <= nums[i], k <= nums.length
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Intuition
Combine results from subarrays with atmost k diferent integers
atmost(k) - atmost(k - 1)

track subarrays with atmost k distinct with two pointers, and freq hashmap

simply add element at hi
while (map.size() > k) - strictly greater, because == is allowed
remove, increment lo

ans += number of elements between hi and lo inclusive, hi - lo + 1
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Time = O(N) - two linear scans
Space = O(K) - hashmap
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