860. Lemonade Change

class Solution {
public:
    bool lemonadeChange(vector<int>& bills)
    {
        int coin5 = 0;
        int coin10 = 0;

        for (const auto& b : bills)
        {
            if (b == 5)
            {
                coin5++;
            }
            else if (b == 10)
            {
                if (coin5 > 0)
                {
                    coin5--;
                    coin10++;
                }
                else return false;
            }
            else if (b == 20)
            {
                if (coin10 > 0 && coin5 > 0)
                {
                    coin5--;
                    coin10--;
                }
                else if (coin5 >= 3)
                {
                    coin5 -= 3;
                }
                else return false;
            }
        }
        return true;
    }
};

• T: $O(n)$
• S: $O(1)$