'''

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.


Note:

0 <= bills.length <= 10000
bills[i] will be either 5, 10, or 20.

'''
import unittest


class Solution:
    def lemonadeChange(self, bills):
        """
        :type bills: List[int]
        :rtype: bool
        """
        five = ten = 0

        for bill in bills:
            if bill == 5:
                five += 1
            elif bill == 10 and five > 0:
                five -= 1
                ten += 1
            elif bill == 20 and five > 0 and ten > 0:
                five -= 1
                ten -= 1
            elif bill == 20 and five > 2:
                five -= 3
            else:
                return False
        return True



class TestSolution(unittest.TestCase):

    def test_case(self):
        examples = (
            ([5,5,5,10,20], True),
            ([5,5,10], True),
            ([10,10], False),
            ([5,5,10,10,20], False)
        )
        for first, second in examples:
            self.assert_function(first, second)

    def assert_function(self, first, second):
        self.assertEqual(Solution().lemonadeChange(first), second,
                         msg="first: {}; second: {}".format(first, second))


unittest.main()
