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J.P. Morgan iOS Developer Interview

The 15 Questions That Nearly Broke Me

Round 1: Online Assessment (HackerRank — 90 minutes)

This round was purely code and fundamentals. Two main coding problems + 10 multiple-choice questions on Swift, iOS, and general CS.

DSA Question 1: Fraud Detection in Transactions

Problem:
You receive a list of transactions with (cardNumber, country, timestamp). Detect if the same card is used in two different countries within 5 minutes.

Thought Process:

1. Sort transactions by timestamp.
2. Store last transaction in a dictionary keyed by card number.
3. Compare current with last — if countries differ and time gap ≤ 5 minutes, return true.

Swift Solution:

struct Transaction {
    let card: String
    let country: String
    let time: Date
}

func detectFraud(transactions: [Transaction]) -> Bool {
    var lastUse: [String: (String, Date)] = [:]
    let sortedTransactions = transactions.sorted { $0.time < $1.time }
    
    for t in sortedTransactions {
        if let (lastCountry, lastTime) = lastUse[t.card] {
            let diff =…