2563. Count the Number of Fair Pairs

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

class Solution {
public:
long long countFairPairs(vector<int>& nums, int lower, int upper) {

ranges::sort(nums);
return countLess(nums, upper) - countLess(nums, lower - 1);
}

private:
long countLess(const vector<int>& nums, int sum) {
long res = 0;
for (int i = 0, j = nums.size() - 1; i < j; ++i) {
while (i < j && nums[i] + nums[j] > sum)
--j;
res += j - i;
}
return res;
}
};