Approach

# 模擬購買
Simulation.

# 如果顧客付20塊錢的話，我會優先找10塊錢(如果有的話)
If the customer pays with a 20, I will give them a 10 first.

# 接著是找3個5塊錢
Then give three 5-dollar coins.

Time Complexity

# n = number of bills
O(n)

Space Complexity

# n = number of bills
O(n)

Code

class Solution {
public:
    bool lemonadeChange(vector<int>& bills) {
        int l = bills.size(), box_5 = 0, box_10 = 0;

        for (int i = 0 ; i < l ; i++) {
            if (bills[i] == 5) {
                box_5++;
            }
            else if (bills[i] == 10) {
                if (box_5 == 0) {
                    return false;
                }
                else {
                    box_5--;
                    box_10++;
                }
            }
            else {
                if (box_10 >= 1 && box_5 >= 1) {
                    box_10--;
                    box_5--;
                }
                else if (box_5 >= 3) {
                    box_5 -= 3;
                }
                else {
                    return false;
                }
            }
        }

        return true;
    }
};