lemonade-change.py

# 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