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 i^th 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.

Submitted Code

class Solution(object):
    def lemonadeChange(self, bills):
        """
        :type bills: List[int]
        :rtype: bool
        """
        five = 0                            # 5$ 개수
        ten = 0                             # 10$ 개수

        for b in bills:
            if b == 5:                      # no change
                five += 1                   
            elif b == 10:
                if five > 0:                
                    five -= 1               # change = 5x1
                    ten += 1
                else:
                    return False
            elif b == 20:
                if five > 0 and ten > 0:    
                    five -= 1               # change = 10x1 + 5x1
                    ten -= 1
                elif five > 2:              
                    five -= 3               # change = 5x3
                else:
                    return False

        return True

Runtime: 0 ms | Beats 100.00%
Memory: 15.17 MB | Beats 89.63%

15달러를 거슬러줘야할 때 단위가 큰 10달러부터 우선 소진하도록 조건을 설정했다.

Other Solutions

1st

    def lemonadeChange(self, bills):
        five = ten = 0
        for i in bills:
            if i == 5: five += 1
            elif i == 10: five, ten = five - 1, ten + 1
            elif ten > 0: five, ten = five - 1, ten - 1
            else: five -= 3
            if five < 0: return False
        return True

time complexity: 𝑂(𝑛)
space complexity: 𝑂(1)

if문 조건을 바꿔서 길이를 좀 더 짧게 만든 코드도 참고했다.