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Problem Description
At the lemonade stand, each glass of lemonade sells for $5.
Customers line up to buy your products, (in the order of bills paid) one cup at a time.
Each customer only buys a glass of lemonade, and then pay you $5, $10 or $20 . You must give each customer the correct change, which means that the net transaction is that each customer pays you $5.
Note that you don't have any change in hand at the beginning.
If you can give each customer the correct change, return true, otherwise return false.
Example 1:
Input : [5,5,5,10,20]
Output : true
Explanation :
From the first 3 customers, we collect 3 5 USD bills in order.
From the 4th customer, we received a $10 bill and returned $5.
For the fifth customer, we returned a 10 dollar bill and a 5 dollar bill.
Since all customers got the 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 2 customers, we collect 2 US$5 bills in order.
For the next 2 customers, we charge a $10 bill and return $5.
For the last customer, we cannot return $15 because we now only have two $10 bills.
Since not every customer gets the correct change, the answer is false.
prompt:
-
0 <= bills.length <= 10000
-
bills[i] is either 5 or 10 or 20
problem analysis
This question can be regarded as a very common question in life. For every customer, if we have enough change to give him change, then we return true, as long as a customer does not have enough change to give him change, we return false.
Customers can only have 3 kinds of banknotes , 5 yuan, 10 yuan, and 20 yuan . We need to count the amount of 5 yuan and 10 yuan, and 20 yuan does not need to be counted, because 20 yuan can’t be given to others.
-
The customer gives 5 yuan, plus 1 for the amount of 5 yuan
-
The customer gives 10 yuan, and the amount of 5 yuan is subtracted by 1 (after the reduction, the amount of 5 yuan will be judged. If it is less than 0, it means that the 5 yuan is not enough, and the customer cannot be changed. Return false directly)
-
The customer gives 20 yuan. According to the common sense of life, if there is a 10 yuan, you should first find him 10 yuan, and then find him a 5 yuan. If there is no 10 yuan, find him 3 5 yuan, and then judge the amount of 5 yuan, if it is less than 0, return false directly.
The principle is relatively simple, let's look at the code
public boolean lemonadeChange(int[] bills) {
//统计店员所拥有的5元和10元的数量（20元的不需要统计，
//因为顾客只能使用5元，10元和20元，而20元是没法
// 给顾客找零的）
int five = 0, ten = 0;
for (int bill : bills) {
if (bill == 5) {
//如果顾客使用的是5元，不用找零，5元数量加1
five++;
} else if (bill == 10) {
//如果顾客使用的是10元，需要找他5元，所以
//5元数量减1,10元数量加1
five--;
ten++;
} else if (ten > 0) {
//否则顾客使用的只能是20元，顾客使用20元的时候，
//如果我们有10元的，要尽量先给他10元的，然后再
//给他5元的，所以这里5元和10元数量都要减1
ten--;
five--;
} else {
//如果顾客使用的是20元，而店员没有10元的，
//就只能给他找3个5元的，所以5元的数量要减3
five -= 3;
}
//上面我们找零的时候并没有判断5元的数量，如果5元的
//数量小于0，说明上面某一步找零的时候5元的不够了，
//也就是说没法给顾客找零，直接返回false即可
if (five < 0) {
return false;
}
}
return true;
}
to sum up
A common sense question in life, when we found change, we did not first judge the amount of 5 yuan, and after the change, we judge whether the amount of 5 yuan is greater than 0 or less than 0. If you first judge the amount of 5 yuan when making change, it's just a little troublesome, because customers must judge first as long as they don't give 5 yuan.