890. Lemonade Change

• Difficulty: Easy

• Topics: Greedy

• Similar Questions:

Problem:

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 don't have any change in hand at first.

Return true if and only if you can provide every customer with correct change.

Input: [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.

Input: [5,5,10]
Output: true

Input: [10,10]
Output: false

Input: [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't give change of $15 back because we only have two $10 bills.
Since not every customer received correct change, the answer is false.

Solutions:

class Solution {
public:
    bool lemonadeChange(vector<int>& bills) {
        map<int, int> changes = {
            {5, 0},
            {10, 0},
            {20, 0}
        };

        for (auto& bill : bills) {
            int change = bill - 5;
            ++changes[bill];
            while (change > 0) {
                for (auto it = changes.rbegin(); ; ++it) {
                    if (it == changes.rend())    return false;
                    if (it->second > 0 && it->first <= change) {
                        --it->second;
                        change -= it->first;
                        break;
                    }
                }
            }
        }

        return true;
    }
};