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.

class Solution {
public:
    bool lemonadeChange(vector<int>& bills) {
        int b5 = 0, b10 = 0;
        for(auto& b : bills) {
            if(b == 5) {
                b5++;
            } else if(b == 10) {
                if(!b5) return false;
                b5--;
                b10++;
            } else {
                if(b10 and b5) {
                    b10--;
                    b5--;
                } else if(b5 >= 3) {
                    b5 -= 3;
                } else return false;
            }
        }
        
        return true;
    }
};