<p>At a lemonade stand, each lemonade costs <code>$5</code>. 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 <code>$5</code>, <code>$10</code>, or <code>$20</code> bill. You must provide the correct change to each customer so that the net transaction is that the customer pays <code>$5</code>.</p>

<p>Note that you do not have any change in hand at first.</p>

<p>Given an integer array <code>bills</code> where <code>bills[i]</code> is the bill the <code>i<sup>th</sup></code> customer pays, return <code>true</code> <em>if you can provide every customer with the correct change, or</em> <code>false</code> <em>otherwise</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> bills = [5,5,5,10,20]
<strong>Output:</strong> true
<strong>Explanation:</strong> 
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.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> bills = [5,5,10,10,20]
<strong>Output:</strong> false
<strong>Explanation:</strong> 
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.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= bills.length &lt;= 10<sup>5</sup></code></li>
	<li><code>bills[i]</code> is either <code>5</code>, <code>10</code>, or <code>20</code>.</li>
</ul>

class Solution:
    def lemonadeChange(self, bills: List[int]) -> bool:
        # greedy algorithm
        my_bill_counts = {5:0, 10:0, 20:0} 
        for bill in bills:
            my_bill_counts[bill] += 1 # increase count of received bill
            needed_change = bill - 5
            print(f'need change: {needed_change}. bills: {my_bill_counts}')

            # get greedy
            while needed_change >= 20 and my_bill_counts[20] > 0:
                my_bill_counts[20] -= 1
                needed_change -= 20
                print(f'gave 20')
            
            while needed_change >= 10 and my_bill_counts[10] > 0:
                my_bill_counts[10] -= 1
                needed_change -= 10
                print(f'gave 10')

            while needed_change >= 5 and my_bill_counts[5] > 0:
                my_bill_counts[5] -= 1
                needed_change -= 5
                print(f'gave 5')
            
            if needed_change != 0:
                print(f'fail. {needed_change}. bills: {my_bill_counts}')
                return False
            else:
                print(f'got correct change: bills: {my_bill_counts}')
        
        return True
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