Posted by Jae on June 19, 2019

### 1、题目

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.

Note:

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

### 3、实现

public class Solution {
// 存放已收到钞票数量
int[] changeArr = new int[21];
// 钞票面值
int[] faceValue = {20, 10, 5};

for (int bill : bills){
int change = bill - 5;
changeArr[bill]++;
if (change > 0){
for (int v : faceValue){
if (change >= v && changeArr[v] > 0){
int count = change / v;
if (changeArr[v] < count){
return false;
}else{
changeArr[v]-=count;
change = change - v * count;
}
}
}
if (change != 0){
return false;
}
}
}
return true;
}
}