2563. Count the Number of Fair Pairs

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2563. Count the Number of Fair Pairs

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:

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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:

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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 <= 10^5
  • nums.length == n
  • -10^9<= nums[i] <= 10^9
  • -10^9<= lower <= upper <= 10^9

Hints/Notes

Solution

Language: C++

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class Solution {
public:
    long long countFairPairs(vector<int>& nums, int lower, int upper) {
        ranges::sort(nums);
        int n = nums.size();
        long long res = 0;
        for (int i = 0; i < n - 1; i++) {
            if (nums[i] + nums[i + 1] > upper) {
                break;
            }
            int mi = lower - nums[i], mx = upper - nums[i];
            auto low = lower_bound(nums.begin() + i + 1, nums.end(), mi);
            auto high = --lower_bound(nums.begin() + i + 1, nums.end(), mx + 1);
            res += (high - low + 1);
        }
        return res;
    }
};