Lemonade Stand

Challenge Difficulty: Medium | Estimated completion time: ~25 minutes

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 need to give each customer the right amount of change so that they end up paying $5.

• For $5 bills: No change needed
• For $10bills:Youneedtogive$5 back
• For $20bills:Youneedtogive$15 back (best using one $10andone$5, or three $5 bills)

Return true if and only if you can provide every customer with correct change.

Examples

lemonade([5, 5, 5, 10, 20])
""" First three customers pay with $5: Now you have three $5 bills
Fourth customer pays $10: You give $5 change, now have two $5 bills and one $10 bill
Fifth customer pays $20: You can give $15 change (one $10 + one $5)
"""
output = True
 
lemonade([5, 5, 10, 10, 20])
""" First two customers pay with $5: Now you have two $5 bills
Third customer pays $10: You give $5 change, now have one $5 bill and one $10 bill
Fourth customer pays $10: You give $5 change, now have zero $5 bills and two $10 bills
Fifth customer pays $20: You need to give $15 change but you can't because you only have $10 bills!
"""
output = False
 
lemonade([10, 10])
output = False
 
lemonade([5, 5, 10])
output = True

Notes

• You don’t have any change in hand at first.
• bills is an integer array.
• bills[i] will be either 5, 10, or 20.

Solution

Python

def lemonade(bills: list[int]) -> bool: """Check if you can provide correct change to all customers. Args: bills: List of bills customers pay with (5, 10, or 20) Returns: True if all customers can be given correct change """ five_count = 0 ten_count = 0 for bill in bills: if bill == 5: five_count += 1 elif bill == 10: if five_count < 1: return False five_count -= 1 ten_count += 1 elif bill == 20: if ten_count >= 1 and five_count >= 1: ten_count -= 1 five_count -= 1 elif five_count >= 3: five_count -= 3 else: return False return True if __name__ == "__main__": print(lemonade([5, 5, 5, 10, 20])) # True print(lemonade([5, 5, 10, 10, 20])) # False print(lemonade([10, 10])) # False print(lemonade([5, 5, 10])) # True

Output