860.swift

//
//  860.swift
//  LeetCodeTests
//
//  Created by Lex on 2018/7/8.
//  Copyright © 2018 Lex Tang. All rights reserved.
//

//  At a lemonade stand, each lemonade costs $5.
//
//  Customers are standing in a queue to buy from you, and order one at a time (in the order specified by bills).
//
//  Each customer will only buy one lemonade and pay with either a $5, $10, or $20 bill.  You must provide the correct change to each customer, so that the net transaction is that the customer pays $5.
//
//  Note that you don't have any change in hand at first.
//
//  Return true if and only if you can provide every customer with correct change.
//
//
//
//  Example 1:
//
//  Input: [5,5,5,10,20]
//  Output: true
//  Explanation:
//  From the first 3 customers, we collect three $5 bills in order.
//  From the fourth customer, we collect a $10 bill and give back a $5.
//  From the fifth customer, we give a $10 bill and a $5 bill.
//  Since all customers got correct change, we output true.
//
//  Example 2:
//
//  Input: [5,5,10]
//  Output: true
//
//  Example 3:
//
//  Input: [10,10]
//  Output: false
//
//  Example 4:
//
//  Input: [5,5,10,10,20]
//  Output: false
//  Explanation:
//  From the first two customers in order, we collect two $5 bills.
//  For the next two customers in order, we collect a $10 bill and give back a $5 bill.
//  For the last customer, we can't give change of $15 back because we only have two $10 bills.
//  Since not every customer received correct change, the answer is false.

import XCTest

func lemonadeChange(_ bills: [Int]) -> Bool {
    var fives = 0
    var tens = 0
    var twenties = 0

    enum ChangeError: Error {
        case cantChange
    }

    func getBill(_ bill: Int) throws {
        switch bill {
        case 5:
            fives += 1

        case 10:
            tens += 1
            if fives > 0 {
                fives -= 1
            } else {
                throw ChangeError.cantChange
            }

        case 20:
            twenties += 1
            if fives >= 3 {
                fives -= 3
            } else if tens >= 1 && fives >= 1 {
                tens -= 1
                fives -= 1
            } else {
                throw ChangeError.cantChange
            }
        default: ()
        }
    }

    do {
        try bills.forEach(getBill)
        return true
    } catch {
        return false
    }
}

class LemonadeChangeTest: XCTestCase {

    func testLemonadeChange() {
        XCTAssertTrue(lemonadeChange([5, 5, 5, 10, 20]))
        XCTAssertTrue(lemonadeChange([5, 5, 10]))
        XCTAssertFalse(lemonadeChange([10, 10]))
        XCTAssertFalse(lemonadeChange([5, 5, 10, 10, 20]))
    }

}