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2563. Count the Number of Fair Pairs

Difficulty Topics

Description

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

count-the-number-of-fair-pairs.py
from sortedcontainers import SortedList

class Solution:
    def countFairPairs(self, nums: List[int], lower: int, upper: int) -> int:
        res = 0
        sl = SortedList()

        for x in nums:
            left = sl.bisect_left(lower - x)
            right = sl.bisect(upper - x)
            res += right - left

            sl.add(x)

        return res