Count the Number of Fair Pairs Python Solution

lcpy gen -n 2563   # by problem number
lcpy gen -s count_the_number_of_fair_pairs   # by problem name

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

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

class Solution:
    # Time: O(n log n)
    # Space: O(n) for the sort
    def count_fair_pairs(self, nums: list[int], lower: int, upper: int) -> int:
        nums.sort()

        def count_at_most(bound: int) -> int:
            left, right = 0, len(nums) - 1
            total = 0
            while left < right:
                if nums[left] + nums[right] <= bound:
                    total += right - left
                    left += 1
                else:
                    right -= 1
            return total

        return count_at_most(upper) - count_at_most(lower - 1)