Lemonade Change Python Solution with Tests

lcpy gen -n 860   # by problem number
lcpy gen -s lemonade_change   # by problem name

Input: bills = [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.

Input: bills = [5,5,10,10,20]
Output: false

class Solution:
    # Time: O(n)
    # Space: O(1)
    def lemonade_change(self, bills: list[int]) -> bool:
        fives = 0
        tens = 0
        for bill in bills:
            if bill == 5:
                fives += 1
            elif bill == 10:
                if fives == 0:
                    return False
                fives -= 1
                tens += 1
            else:  # bill == 20, prefer giving a $10 + $5 to conserve $5 bills
                if tens > 0 and fives > 0:
                    tens -= 1
                    fives -= 1
                elif fives >= 3:
                    fives -= 3
                else:
                    return False
        return True