2563. Count the Number of Fair Pairs

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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],

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