# [Greedy, Classic]
# https://leetcode.com/problems/lemonade-change/
# 860. 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 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
        """
        changes = {
            5: 0,
            10: 0,
            20: 0
        }

        for b in bills:
            change = b - 5
            if change == 0:
                changes[5] += 1
            if change == 5:
                if changes[5] <= 0:
                    return False
                changes[5] -= 1
                changes[10] += 1
            if change == 15:
                if changes[10] >= 1:
                    change -= 10
                    changes[10] -= 1

                five_needed = change / 5
                if changes[5] < five_needed:
                    return False
                changes[5] -= five_needed
                changes[20] += 1

        return True
