Lemonade Change

Description

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 don't have any change in hand at first.

Return true if and only if you can provide every customer with correct change.

 

Example 1:

Input: [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: [5,5,10]
Output: true

Example 3:

Input: [10,10]
Output: false

Example 4:

Input: [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't give change of $15 back because we only have two $10 bills.
Since not every customer received correct change, the answer is false.

 

Note:

• 0 <= bills.length <= 10000
• bills[i] will be either 5, 10, or 20.

Solution(javascript)

const lemonadeChange = (bills = []) => {
  let fives = 0
  let tens = 0
  for (let i = 0; i < bills.length; i++) {
    switch (bills[i]) {
      case 5:
        fives += 1
        break
      case 10:
        if (fives <= 0) {
          return false
        }
        fives -= 1
        tens += 1
        break
      default:
        if (tens >= 1 && fives >= 1) {
          tens -= 1
          fives -= 1
          // twenties += 1
        } else if (fives >= 3) {
          fives -= 3
        } else {
          return false
        }
        break
    }
  }
  return true
}