Problem

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 ith customer pays, return true if you can provide every customer with the correct change, or false otherwise.

Examples

Example 1:

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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:

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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

Method 1 - Count 5 and 10 bills

When the customer gives us $20, we have two options:

  1. Return back three $5, OR
  2. Return back one $5 and one $10.

When possible, second option is better because this way we have two $5 bills, which we can use in future transactions. So, this is why this is a greedy option, where we greedily try to return back $10s, and then use $5.

So, we can keep track of number of $5 and $10 bills. We don’t have to keep track of $20 bills, because customer will buy 1 lemonade at least, and we will never return them back $20 bills. So, we don’t have to track $20 bills to check if change can be made or not.

Video

Here is the video explanation:

Code

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class Solution {
    public boolean lemonadeChange(int[] bills) {
        int five = 0, ten = 0;
        for (int bill : bills) {
            if (bill == 5) {
                five++;
            } else if (bill == 10) {
                ten++;
                if (five > 0) {
                    five--;
                } else {
                    return false;
                }
            } else { // b == 20
                if (ten > 0 && five > 0) {
                    ten--;
                    five--;
                } else if (five > 2) {
                    five -= 3;
                } else {
                    return false;
                }
            }
        }
        return true;
    }
}

Complexity

  • ⏰ Time complexity: O(n) where is number of transactions
  • 🧺 Space complexity: O(1)