type Transaction struct {
    city   string
    time   int
    amount int
    index  int
    valid  bool
}

func invalidTransactions(transactions []string) []string {
    // key is a name of an owner, value is an array of transactions
    nameTransactionMap := map[string][]Transaction{}

    // construct the map
    for i, tran := range transactions {
        info := strings.Split(tran, ",")
        time, _ := strconv.Atoi(info[1])
        amount, _ := strconv.Atoi(info[2])
        nameTransactionMap[info[0]] = append(nameTransactionMap[info[0]], transaction{
            city:  info[3],
            time:  time,
            amount: amount,
            index: i,
            valid: amount <= 1000,
        })
    }

    invalidTransactions := []string{}

    for name := range nameTransactionMap {
        // sort by time to avoid time miscalculation
        sort.Slice(nameTransactionMap[name], func(x, y int) bool {
            return nameTransactionMap[name][x].time < nameTransactionMap[name][y].time
        })

        // add to invalidTransactions based on isValid function's result
        for i := range nameTransactionMap[name] {
            if !isValid(nameTransactionMap[name], i) {
                invalidTransactions = append(invalidTransactions, transactions[nameTransactionMap[name][i].index])
            }
        }
    }
    return invalidTransactions
}

func isValid(list []transaction, index int) bool {
    if list[index].valid == false {
        return false
    }

    // compare previous transactions with the current
    for i := index-1; i >= 0; i-- {
        if list[index].time - list[i].time > 60 {
            break
        }
        if list[index].city != list[i].city {
            list[index].valid = false
            list[i].valid = false
        }
    }

    // compare next transactions with the current
    for i := index + 1; i < len(list); i++ {
        if list[i].time - list[index].time > 60 {
            break
        }
        if list[index].city != list[i].city {
            list[index].valid = false
            list[i].valid = false
        }
    }
    
    return list[index].valid
}