2537. Count the Number of Good Subarrays

2537. Count the Number of Good Subarrays

Difficulty: Medium

Topics: Array, Hash Table, Sliding Window

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

Hint:

1. For a fixed index l, try to find the minimum value of index r, such that the subarray is not good
2. When a number is added to a subarray, it increases the number of pairs by its previous appearances.
3. When a number is removed from the subarray, it decreases the number of pairs by its remaining appearances.
4. Maintain 2-pointers l and r such that we can keep in account the number of equal pairs.

Solution:

We need to count the number of good subarrays in a given array. A good subarray is defined as one where there are at least k pairs of indices (i, j) such that i < j and arr[i] == arr[j].

Approach

The approach to solve this problem efficiently involves using a sliding window technique combined with a hash map to track the frequency of elements within the current window. Here are the key steps:

1. Sliding Window Technique: Maintain a window using two pointers, left and right, to explore all possible subarrays.
2. Frequency Tracking: Use a hash map to keep track of the frequency of each element within the current window. This helps in efficiently calculating the number of pairs contributed by each element.
3. Sum of Pairs Calculation: For each element added to the window, the number of pairs increases by the current frequency of that element before it was added. Similarly, when an element is removed from the window, the number of pairs decreases by the new frequency of that element after removal.
4. Expand and Shrink Window: Expand the window by moving the right pointer to include more elements until the number of pairs meets or exceeds k. Once the condition is met, count all valid subarrays ending at the current right pointer and shrink the window by moving the left pointer to find smaller valid subarrays.

Let's implement this solution in PHP: 2537. Count the Number of Good Subarrays

<?php
/**
 * @param Integer[] $nums
 * @param Integer $k
 * @return Integer
 */
function countGood($nums, $k) {
    ...
    ...
    ...
    /**
     * go to ./solution.php
     */
}

// Example 1
$nums1 = array(1,1,1,1,1);
$k1 = 10;
echo countGood($nums1, $k1) . "\n"; // Output: 1

// Example 2
$nums2 = array(3,1,4,3,2,2,4);
$k2 = 2;
echo countGood($nums2, $k2) . "\n"; // Output: 4
?>

Explanation:

1. Initialization: Initialize pointers left and right at the start of the array, a hash map frequency to track element frequencies, sum_pairs to track the number of valid pairs, and result to accumulate the count of good subarrays.
2. Expand Window: Move the right pointer to expand the window. For each new element added, update its frequency and adjust sum_pairs by adding the previous frequency of that element (since each existing occurrence forms a new pair).
3. Count Valid Subarrays: Once the window contains enough pairs (sum_pairs >= k), all subarrays starting from the current left to the current right (and beyond) are valid. Add the count of these valid subarrays to result.
4. Shrink Window: Move the left pointer to shrink the window, adjusting the frequency and sum_pairs accordingly. This ensures that we explore all possible valid subarrays starting from each position.

This approach efficiently counts all valid subarrays in linear time, O(n), by ensuring each element is processed at most twice (once added and once removed from the window).

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