Lemonade Change
<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>
Greedy; Array
function lemonadeChange(bills: number[]): boolean {
    let [five, ten] = [0, 0];
    for (const x of bills) {
        switch (x) {
            case 5:
                five++;
                break;
            case 10:
                five--;
                ten++;
                break;
            case 20:
                if (ten) {
                    ten--;
                    five--;
                } else {
                    five -= 3;
                }
                break;
        }

        if (five < 0) {
            return false;
        }
    }
    return true;
}
