860 - Lemonade Change

#easy

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.

class Solution {
public:
    bool lemonadeChange(vector<int>& bills) {
        int five_d = 0;
        int ten_d = 0;
        for ( int i = 0; i < bills.size(); i++ ) {
            if ( bills[i] == 5 ) five_d++;
            else if ( bills[i] == 10 ) {
                if ( five_d < 1 ) return false;
                else {
                    ten_d++;
                    five_d--;
                }
            }
            else if ( bills[i] == 20 ) {
                if (  five_d >= 1 && ten_d >= 1) {
                    five_d--;
                    ten_d--;
                }
                else if ( five_d >=3 ) {
                    five_d -= 3;
                }
                else return false;
            }

        }

        return true;
    }
};