860. 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 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.

 

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.

 

Constraints:

• 1 <= bills.length <= 105
• bills[i] is either 5, 10, or 20.

 

Solutions

Solution: Greedy

• Time complexity: O(n)
• Space complexity: O(1)

 

JavaScript

/**
 * @param {number[]} bills
 * @return {boolean}
 */
const lemonadeChange = function (bills) {
  const COST = 5;
  let five = 0;
  let ten = 0;

  for (const bill of bills) {
    const needBack = bill - COST;

    if (needBack === 5) {
      if (!five) return false;
      five -= 1;
    }
    if (needBack === 10) {
      if (!ten && five < 2) return false;
      ten ? (ten -= 1) : (five -= 2);
    }
    if (needBack === 15) {
      if ((!ten || !five) && five < 3) return false;
      if (ten && five) {
        ten -= 1;
        five -= 1;
      } else five -= 3;
    }
    if (bill === 5) five += 1;
    if (bill === 10) ten += 1;
  }
  return true;
};