## Problem You have an array `prices` of item prices and a discount `d`. A "discount pair" is a pair of indices `(i, j)` where `i int: ``` ## Example ``` prices = [1, 3, 2, 4, 5], d = 6 Valid pairs (0-indexed): (0,1)=4, (0,2)=3, (0,3)=5, (0,4)=6, (1,2)=5, (2,3)=6 Output: 6 ``` Brute force is O(n^2). A two-pointer approach after sorting runs in O(n log n). ## Follow-ups 1. After sorting, walk through the two-pointer approach -- why is it correct even though sorting changes original indices? 2. What if the condition is `prices[i] * prices[j] <= d` instead of sum? Does two-pointer still work? 3. Count pairs where the discount is between `[lo, hi]` inclusive (i.e., `lo <= prices[i] + prices[j] <= hi`). 4. How would you parallelize the counting over a very large `prices` array?

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Count Discount Pairs - Count Item Pairs with Combined Discount Above Threshold

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Problem You have an array prices of item prices and a discount d. A "discount pair" is a pair of indices (i, j) where i < j and prices[i] + prices[j] <= d. Return the count of all such pairs. Example Brute force is O(n^2). A two-pointer approach afte…

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