2563

2563. Count the Number of Fair Pairs (Medium)

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

Companies: Salesforce

Related Topics:
Array, Two Pointers, Binary Search, Sorting

Similar Questions:

• Count of Range Sum (Hard)
• Finding Pairs With a Certain Sum (Medium)
• Count Number of Pairs With Absolute Difference K (Easy)

Solution 1.

// OJ: https://leetcode.com/problems/count-the-number-of-fair-pairs
// Author: github.com/lzl124631x
// Time: O(NlogN)
// Space: O(N)
class Solution {
public:
    long long countFairPairs(vector<int>& A, int lower, int upper) {
        sort(begin(A), end(A));
        long long N = A.size(), ans = 0;
        for (int i = 0; i < N; ++i) {
            ans += upper_bound(begin(A) + i + 1, end(A), upper - A[i]) - lower_bound(begin(A) + i + 
1, end(A), lower - A[i]);
        }
        return ans;
    }
};