Count The Number Of Fair Pairs · Interview Problem

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Leetcode

Medium

Arrays

Sorting

Two Pointers

Amazon

Count The Number Of Fair Pairs

Count the number of index pairs whose sum lies within an inclusive range.

Acceptance 0%

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Problem Statement

Problem

Given an integer array nums and two integers lower and upper, count the number of fair pairs of indices (i, j) such that:

0 <= i < j < nums.length
lower <= nums[i] + nums[j] <= upper

Return the total number of such pairs.

A pair is considered fair if its sum falls within the inclusive range [lower, upper].

Notes

Each index may be used in at most one pair occurrence, but the same value can appear multiple times at different indices.
Count pairs by index, not by value.
The answer may be large, so use an integer type that can hold the full count.

Input Format

An integer array nums
Two integers lower and upper

Output Format

Return one integer: the number of pairs (i, j) with i < j and lower <= nums[i] + nums[j] <= upper.

Constraints

1 <= nums.length
Pair counting is based on indices, not distinct values
lower <= upper
Values may be negative, zero, or positive
The result may exceed 32-bit integer range

Examples

Sample cases returned by the problem API.

Example 1

Input

nums = [0,1,7,4,4,5], lower = 3, upper = 6

Output

Explanation

The fair pairs are the index pairs whose sums are 3, 4, 5, or 6. There are 6 such pairs in total.

Example 2

Input

nums = [1,7,9,2,5], lower = 11, upper = 11

Output

Explanation

Only one pair sums to exactly 11: (2, 9).

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