2537. Count the Number of Good Subarrays

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 subarray arr is good if there are at least k pairs of indices (i, j) such that i < j and arr[i] == arr[j].

A subarray is a contiguous non-empty sequence of elements within an array.

 

Example 1:

Input: nums = [1,1,1,1,1], k = 10
Output: 1
Explanation: The only good subarray is the array nums itself.

Example 2:

Input: nums = [3,1,4,3,2,2,4], k = 2
Output: 4
Explanation: There are 4 different good subarrays:
- [3,1,4,3,2,2] that has 2 pairs.
- [3,1,4,3,2,2,4] that has 3 pairs.
- [1,4,3,2,2,4] that has 2 pairs.
- [4,3,2,2,4] that has 2 pairs.

 

Constraints:

• 1 <= nums.length <= 105
• 1 <= nums[i], k <= 109

Solution

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Python3

class Solution:
    def countGood(self, nums: List[int], k: int) -> int:
        N = len(nums)
        cnt = defaultdict(int)
        pairs = 0
        res = 0
        i = 0
        
        for j, x in enumerate(nums):
            pairs += cnt[x]
            cnt[x] += 1
            
            while pairs >= k:
                res += (N - j)
                pairs -= cnt[nums[i]] - 1
                cnt[nums[i]] -= 1
                i += 1
        
        return res