# leetcode\_860

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.

Example 1:

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. Example 2:

Input: \[5,5,10] Output: true Example 3:

Input: \[10,10] Output: false Example 4:

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.

Note:

0 <= bills.length <= 10000 bills\[i] will be either 5, 10, or 20.

## Solutions

1. **greedy strategy**

```cpp
class Solution {
public:
    bool lemonadeChange(vector<int>& bills) {
        int n5 = 0, n10 = 0;
        for (auto n : bills) {
            if (n == 5)
                n5++;
            else if (n == 10) {
                if (!n5) return false;
                n10++; n5--;
            }
            else if (n == 20) {
                n = 15;
                if (n10) {
                    n10--; n -= 10;
                }
                n5 -= n / 5;
                if (n5 < 0) return false;
            }
        }

        return true;
    }
};
```