Formatted question description: https://leetcode.ca/all/2043.html

# 2043. Simple Bank System (Medium)

You have been tasked with writing a program for a popular bank that will automate all its incoming transactions (transfer, deposit, and withdraw). The bank has n accounts numbered from 1 to n. The initial balance of each account is stored in a 0-indexed integer array balance, with the (i + 1)th account having an initial balance of balance[i].

Execute all the valid transactions. A transaction is valid if:

• The given account number(s) are between 1 and n, and
• The amount of money withdrawn or transferred from is less than or equal to the balance of the account.

Implement the Bank class:

• Bank(long[] balance) Initializes the object with the 0-indexed integer array balance.
• boolean transfer(int account1, int account2, long money) Transfers money dollars from the account numbered account1 to the account numbered account2. Return true if the transaction was successful, false otherwise.
• boolean deposit(int account, long money) Deposit money dollars into the account numbered account. Return true if the transaction was successful, false otherwise.
• boolean withdraw(int account, long money) Withdraw money dollars from the account numbered account. Return true if the transaction was successful, false otherwise.

Example 1:

Input
["Bank", "withdraw", "transfer", "deposit", "transfer", "withdraw"]
[[[10, 100, 20, 50, 30]], [3, 10], [5, 1, 20], [5, 20], [3, 4, 15], [10, 50]]
Output
[null, true, true, true, false, false]

Explanation
Bank bank = new Bank([10, 100, 20, 50, 30]);
bank.withdraw(3, 10);    // return true, account 3 has a balance of $20, so it is valid to withdraw$10.
// Account 3 has $20 -$10 = $10. bank.transfer(5, 1, 20); // return true, account 5 has a balance of$30, so it is valid to transfer $20. // Account 5 has$30 - $20 =$10, and account 1 has $10 +$20 = $30. bank.deposit(5, 20); // return true, it is valid to deposit$20 to account 5.
// Account 5 has $10 +$20 = $30. bank.transfer(3, 4, 15); // return false, the current balance of account 3 is$10,
// so it is invalid to transfer \$15 from it.
bank.withdraw(10, 50);   // return false, it is invalid because account 10 does not exist.


Constraints:

• n == balance.length
• 1 <= n, account, account1, account2 <= 105
• 0 <= balance[i], money <= 1012
• At most 104 calls will be made to each function transfer, deposit, withdraw.

## Solution 1.

// OJ: https://leetcode.com/problems/simple-bank-system/
// Time: O(1) for all
// Space: O(N)
class Bank {
vector<long long> balance;
bool valid(int account) {
return account > 0 && account <= balance.size();
}
bool enough(int account, long long money) {
return balance[account - 1] >= money;
}
public:
Bank(vector<long long>& balance) : balance(balance) {}
bool transfer(int from, int to, long long money) {
if (valid(from) && valid(to) && enough(from, money)) {
balance[from - 1] -= money;
balance[to - 1] += money;
return true;
}
return false;
}
bool deposit(int account, long long money) {
if (valid(account)) {
balance[account - 1] += money;
return true;
}
return false;
}
bool withdraw(int account, long long money) {
if (valid(account) && enough(account, money)) {
balance[account - 1] -= money;
return true;
}
return false;
}
};


We can also implement transfer using withdraw and deposit.

bool transfer(int from, int to, long long money) {
return valid(to) && withdraw(from, money) && deposit(to, money);
}


## Discuss

https://leetcode.com/problems/simple-bank-system/discuss/1525207/C%2B%2B-Straightforward