# Time: O(n) # Space: O(1) # 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(object): def lemonadeChange(self, bills): """ :type bills: List[int] :rtype: bool """ five, ten = 0, 0 for bill in bills: if bill == 5: five += 1 elif bill == 10: if not five: return False five -= 1 ten += 1 else: if ten and five: ten -= 1 five -= 1 elif five >= 3: five -= 3 else: return False return True