@TOC

## # 题目描述

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.

Note:

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

## # 解题方法

class Solution:
"""
:type bills: List[int]
:rtype: bool
"""
changes = {5:0, 10:0}
for bill in bills:
if bill == 5:
changes[5] += 1
elif bill == 10:
if changes[5] == 0:
return False
else:
changes[10] += 1
changes[5] -= 1
elif bill == 20:
if changes[10] != 0:
if changes[5] == 0:
return False
else:
changes[5] -= 1
changes[10] -= 1
else:
if changes[5] < 3:
return False
else:
changes[5] -= 3
return True

## # 日期

2018 年 7 月 4 日 ———— 夏天挺热的，记得吃饭，防止低血糖 2018 年 11 月 13 日 —— 时间有点快