LeetCode-in-Java

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

Solution

import java.util.Arrays;

public class Solution {
    public long countFairPairs(int[] nums, int lower, int upper) {
        Arrays.sort(nums);
        long c1 = 0;
        long c2 = 0;
        int i = 0;
        int j = nums.length - 1;
        while (i < j) {
            if (nums[i] + nums[j] <= upper) {
                c1 = c1 + (j - i);
                i++;
            } else {
                j--;
            }
        }
        i = 0;
        j = nums.length - 1;
        while (i < j) {
            if (nums[i] + nums[j] < lower) {
                c2 = c2 + (j - i);
                i++;
            } else {
                j--;
            }
        }
        return c1 - c2;
    }
}

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