题目：

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代码(首刷自解 2024年2月15日）：

class Solution {
public:
    bool lemonadeChange(vector<int>& bills) {
        vector<int> leftchange(2,0);//leftchange[0]代表5元数量，1代表10
        for (int i = 0; i < bills.size(); ++i) {
            if (bills[i] == 5) leftchange[0]++;
            else if (bills[i] == 10) {
                leftchange[1]++;
                if (leftchange[0] == 0) return false;
                else leftchange[0]--;
            } else {//顾客付了20
                if (leftchange[1] > 0) {
                    leftchange[1]--;
                    if (leftchange[0] == 0) return false;
                    else leftchange[0]--;
                } else {
                    if (leftchange[0] < 3) return false;
                    else leftchange[0] = leftchange[0] - 3;
                }
            }
        }
        return true;
    }
};