2699. Count The Number Of Fair Pairs¶

LeetCode Problem View on GitHub

2699. 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¶

class Solution {
    public long countFairPairs(int[] nums, int lower, int upper) {
        int n = nums.length;
        long count = 0;
        Arrays.sort(nums);
        return solve(nums, upper) - solve(nums, lower - 1);
    }
    private long solve(int arr[], int sum) {
        int n = arr.length;
        long res = 0;
        int start = n - 1;
        for (int i = 0; i < start; i++) {
            while (i < start && arr[i] + arr[start] > sum) start--;
            res += start - i;
        }
        return res;
    }
}

Complexity Analysis¶

• Time Complexity: O(?)
• Space Complexity: O(?)

Approach¶

Detailed explanation of the approach will be added here