class Solution {
public:
    bool lemonadeChange(vector<int>& bills) {
        int x20 = 0, x10 = 0, x5 = 0;
        for (auto b: bills) {
            if (b == 20) {
                // Try 10, 5;  then 5, 5, 5
                if (x10 && x5) {
                    x10--;
                    x5--;
                }
                else if (x5 >= 3) {
                    x5 -= 3;
                }
                else {
                    return false;
                }
                x20++;
            }
            else if (b == 10) {
                if (x5) {
                    x5--;
                }
                else {
                    return false;
                }
                x10++;
            }
            else {
                x5++;
            }
        }
        return true;
    }
};