860 Lemonade Change.py

#!/usr/bin/python3
"""
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.
"""


class Solution:
    def lemonadeChange(self, bills: List[int]) -> bool:
        """
        count
        """
        five, ten, twenty = 0, 0, 0
        for b in bills:
            if b == 5:
                five += 1
            elif b == 10:
                if five < 1:
                    return False
                five -= 1
                ten += 1
            else:  # 20
                if ten >= 1 and five >= 1:
                    ten -= 1  # ten first
                    five -= 1
                elif five >= 3:
                    five -= 3
                else:
                    return False

        return True