860_Lemonade_Change.cpp

/*
860. Lemonade Change
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.
*/
class Solution {
public:
    bool lemonadeChange(vector<int>& bills) {
        int cnt5=0, cnt10=0;
        for(int b: bills){
            switch(b){
                case 5: 
                    cnt5++; 
                    break;
                case 10: 
                    if (cnt5>0) cnt5--, cnt10++;
                    else return 0;
                    break;
                case 20:
                    if (cnt10>0 && cnt5>0) cnt10--, cnt5--;
                    else if (cnt5>2) cnt5-=3;
                    else return 0;
                    break;
            }
        }
        return 1;
    }
};