Tmp Bank Transfer

TransferMain.java

import java.util.*;

public class TmpMain {
    List<One2OneTransfer> makeTransfer(List<AccountBalance> accounts, int baseAmount) {
        if (accounts == null || accounts.isEmpty()) {
            return Collections.emptyList(); // TODO we treat this case as doing nothing
        }
        int sum = accounts.stream().mapToInt(a -> a.balance).sum(); // TODO we assume amount sum would not exceed MAX INT
        if (sum / accounts.size() < baseAmount) {
            return null;    // null means unable to do such a transfer
        }
        List<One2OneTransfer> transfers = new ArrayList<>();
        int high = 0, low = 0;
        while (true) {
            while (high < accounts.size() && accounts.get(high).balance <= baseAmount) ++high;
            while (low < accounts.size() && accounts.get(low).balance >= baseAmount) ++low;
            if (low < accounts.size() && high < accounts.size()) {
                int highExt = accounts.get(high).balance - baseAmount;
                int lowExt = baseAmount - accounts.get(low).balance;
                int amount = Math.min(highExt, lowExt);
                accounts.get(high).balance -= amount;
                accounts.get(low).balance += amount;
                transfers.add(new One2OneTransfer(accounts.get(high).id, accounts.get(low).id, amount));
                if (highExt == lowExt) {
                    ++high;
                    ++low;
                } else if (highExt < lowExt) {
                    ++high;
                } else {
                    ++low;
                }
            } else {
                break;
            }
        }
        return transfers;
    }

    public static void main(String[] args) {
        TmpMain m = new TmpMain();
        List<AccountBalance> accounts = List.of(
                new AccountBalance("a1", 110),
                new AccountBalance("a2", 110),
                new AccountBalance("a3", 90),
                new AccountBalance("a4", 100),
                new AccountBalance("a5", 130),
                new AccountBalance("a6", 70)
        );
        System.out.println(accounts);
        System.out.println(m.makeTransfer(accounts, 100));
    }
}

class AccountBalance {
    String id;
    int balance;

    public AccountBalance(String id, int balance) {
        this.id = id;
        this.balance = balance;
    }

    @Override
    public String toString() {
        return id + ": " + balance;
    }
}

/**
 * transfer from -> to with amount money
 * amount must be positive integer
 */
class One2OneTransfer {
    String from;
    String to;
    int amount;

    public One2OneTransfer(String from, String to, int amount) {
        this.from = from;
        this.to = to;
        this.amount = amount;
    }

    @Override
    public String toString() {
        return from + " -> " + to + " : " + amount;
    }
}