Greedy Algorithm - 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 do not have any change in hand at first.

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

Example 1:

Input: bills = [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: bills = [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 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.

Solution:

/**
 * @param {number[]} bills
 * @return {boolean}
 */
function lemonadeChange(bills) {
    let fiveDollarCount = 0;
    let tenDollarCount = 0;

    for (let bill of bills) {
        if (bill === 5) {
            fiveDollarCount++;
        } else if (bill === 10) {
            if (fiveDollarCount > 0) {
                fiveDollarCount--;
                tenDollarCount++;
            } else {
                return false;
            }
        } else if (bill === 20) {
            if (tenDollarCount > 0 && fiveDollarCount > 0) {
                tenDollarCount--;
                fiveDollarCount--;
            } else if (fiveDollarCount >= 3) {
                fiveDollarCount -= 3;
            } else {
                return false;
            }
        }
    }

    return true;
}

Time Complexity - O(n) (n is the number of customers)

Space Complexity - O(1) (2 variables)

Thank you for reading!