README.md

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

Solutions (Python)

1. Solution

class Solution:
    def countFairPairs(self, nums: List[int], lower: int, upper: int) -> int:
        nums.sort()
        ret = 0

        for i in range(len(nums) - 1):
            if nums[i] + nums[-1] < lower:
                continue
            if nums[i] + nums[i + 1] > upper:
                break

            j = max(bisect.bisect_left(nums, lower - nums[i]), i + 1)
            k = bisect.bisect(nums, upper - nums[i])
            ret += k - j

        return ret