Fraud Pair Match for Chargebacks

Problem Narrative

A large fintech processes tens of millions of card transactions per day. During chargeback investigations, analysts often look for pairs of transactions whose amounts sum to a suspicious threshold (e.g., two split purchases that together hit a reporting limit). You’re building a backend utility that must find such a pair quickly to keep investigation tooling responsive.

Task

Given an integer array amounts (transaction amounts in cents) and an integer target (also in cents), return the indices of the two distinct elements such that:

• amounts[i] + amounts[j] == target
• i != j

Function Signature

• Input: amounts: List[int], target: int
• Output: List[int] of length 2: [i, j]

Requirements

1. If multiple valid answers exist, return the pair with the smallest second index j. If still tied, choose the one with the smallest first index i.
2. If no pair exists, return an empty list [].
3. You may not use the same element twice.

Constraints

• 2 <= amounts.length <= 200_000
• -10^9 <= amounts[i] <= 10^9
• -10^9 <= target <= 10^9

Notes / Clarifications

• Indices are 0-based.
• Negative values are allowed (refunds/adjustments).
• Aim for a solution that scales to hundreds of thousands of elements.