Lemonade Change

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 do not have any change in hand at first.

Given an integer array bills where bills[i] is the bill the ith customer pays, return true if you can provide every customer with the correct change, or false otherwise.

 

Example 1:

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.

Example 2:

Input: bills = [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 not give the change of $15 back because we only have two $10 bills.
Since not every customer received the correct change, the answer is false.

 

Constraints:

• 1 <= bills.length <= 105
• bills[i] is either 5, 10, or 20.

SOLUTION:

from collections import Counter

class Solution:
    def lemonadeChange(self, bills: List[int]) -> bool:
        ctr = Counter()
        for bill in bills:
            ctr[bill] += 1
            if bill == 10:
                if ctr[5] >= 1:
                    ctr[5] -= 1
                else:
                    return False
            elif bill == 20:
                if ctr[10] >= 1 and ctr[5] >= 1:
                    ctr[10] -= 1
                    ctr[5] -= 1
                elif ctr[5] >= 3:
                    ctr[5] -= 3
                else:
                    return False
        return True