992. Subarrays with K Different Integers

Problem Link

Description

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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

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Python3

class Solution:
    def subarraysWithKDistinct(self, nums: List[int], k: int) -> int:
        N = len(nums)
 
        # exact(k) = atMost(k) - atMost(k - 1)
 
        def atMost(k):
            counter = Counter()
            res = i = count = 0
 
            for j, x in enumerate(nums):
                if counter[x] == 0:
                    count += 1
 
                counter[x] += 1
 
                while count > k:
                    counter[nums[i]] -= 1
                    if counter[nums[i]] == 0:
                        count -= 1
                    
                    i += 1
                
                res += j - i - 1
 
            return res
        
        return atMost(k) - atMost(k - 1)