2537.js

/** ------------------------------------------------------------------------------
 * 
 * 2537. Count the Number of Good Subarrays
 * Topics: Hash Table, Sliding Window
 * https://leetcode.com/problems/count-the-number-of-good-subarrays/description/?envType=daily-question&envId=2025-04-16
 * 
 ------------------------------------------------------------------------------ */
/**
 * @param {number[]} nums
 * @param {number} k
 * @return {number}
 */
function countGood(nums, k) {
  let n = nums.length
  let good = 0 // Stores the number of good subarrays
  let freqMap = new Map() // To track the frequency of elements
  let pairCount = 0 // To track how many pairs are present in the current window
  let i = 0 // Left pointer of the sliding window

  for (let j = 0; j < n; j++) {
    // Update the frequency of the current element (nums[j])
    let currentFreq = freqMap.get(nums[j]) || 0
    // Add the new pairs formed with this element
    pairCount += currentFreq

    // Increase the frequency of nums[j] in the map
    freqMap.set(nums[j], currentFreq + 1)

    // Now check if we have enough pairs
    while (pairCount >= k) {
      // All subarrays starting from `i` to `j` are valid
      good += n - j

      // Now shrink the window from the left (increment `i`)
      let leftFreq = freqMap.get(nums[i]) || 0
      pairCount -= leftFreq - 1 // We remove the contribution of nums[i]
      freqMap.set(nums[i], leftFreq - 1)
      i++
    }
  }

  return good
}

console.log(countGood([1, 1, 1, 1, 1], 10))