You sell lemonade for 5, 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 15). Fourth pays 5 back (10). Fifth pays 15 back ($0 left). All served!
What is the greedy choice when giving change? The greedy choice: when you need 10 plus one 5 bills. Why? The $5 is more versatile for future customers.