LeetCode in Kotlin

2563. Count the Number of Fair Pairs

Medium

Given a 0-indexed integer array nums of size n and two integers lower and upper, return the number of fair pairs.

A pair (i, j) is fair if:

• 0 <= i < j < n, and
• lower <= nums[i] + nums[j] <= upper

Example 1:

Input: nums = [0,1,7,4,4,5], lower = 3, upper = 6

Output: 6

Explanation: There are 6 fair pairs: (0,3), (0,4), (0,5), (1,3), (1,4), and (1,5).

Example 2:

Input: nums = [1,7,9,2,5], lower = 11, upper = 11

Output: 1

Explanation: There is a single fair pair: (2,3).

Constraints:

• 1 <= nums.length <= 105
• nums.length == n
• -109 <= nums[i] <= 109
• -109 <= lower <= upper <= 109

Solution

class Solution {
    fun countFairPairs(nums: IntArray, lower: Int, upper: Int): Long {
        nums.sort()
        return smaller(nums, upper) - smaller(nums, lower - 1)
    }

    private fun smaller(nums: IntArray, value: Int): Long {
        var l = 0
        var r = nums.size - 1
        var result: Long = 0
        while (l < r) {
            val sum = nums[l] + nums[r]
            if (sum <= value) {
                result += (r - l).toLong()
                l++
            } else {
                r--
            }
        }
        return result
    }
}

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