Problem - Lemonade Change

You sell lemonade for $5 each. Customers pay with$5, $10, or$20 bills, and you must give correct change. You start with no money. Return true if you can serve all customers.

Example: bills = [5, 5, 5, 10, 20]. Answer: true. First three give $5 each (you have$15). Fourth pays $10, you give$5 back ($10 left as one$10). Fifth pays $20, you give$15 back ($0 left). All served!

What is the greedy choice when giving change? The greedy choice: when you need $15 back, prefer one$10 plus one $5 over three$5 bills. Why? The $5 is more versatile for future customers.