Get Discount Pairs

Problem Overview

• Count pairs (i < j) of products whose prices sum to a multiple of x.
• Input: integer x and array prices of n products; Output: number of valid pairs.
• Constraints: 1 <= x <= 2*10^9, 1 <= n <= 10^5, 1 <= prices[i] <= 9.
• Real-world e-commerce context: discount applies to qualifying product pairs.
• From Amazon interviews; a coding interview problem and common interview question.