一、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 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.

二、题解

class Solution {
public:bool lemonadeChange(vector<int>& bills) {int n = bills.size();vector<int> balance(3,0);for(int i = 0;i < n;i++){int sub = bills[i] - 5;if(sub == 5 && balance[0] == 0) return false;else if(sub == 15 && (balance[0] == 0 || (balance[1] == 0 && balance[0] < 3))) return false;if(sub == 5) balance[0]--;else if(sub == 15){if(balance[1] > 0) balance[0]--,balance[1]--;else balance[0] -= 3;}if(bills[i] == 5) balance[0]++;else if(bills[i] == 10) balance[1]++;else balance[2]++;}return true;}
};

更好的解法

class Solution {
public:bool lemonadeChange(vector<int>& bills) {int n = bills.size();int five = 0,ten = 0,twenty = 0;for(int i = 0;i < n;i++){if(bills[i] == 5) five++;else if(bills[i] == 10){if(five == 0) return false;ten++;five--;}else if(bills[i] == 20){if(five > 0 && ten > 0) five--,ten--;else if(five >= 3 && ten == 0) five -= 3;else return false;}}return true;}
};