礼品卡

#include <vector>
using namespace std;

class GiftCard {
public:
    int balance;

    GiftCard(int start) {
        balance = start;
    }

    bool pay(int amount) {
        // 在这里写：够了就扣钱，返回 true
        return false;
    }
};

int card_left(int start, const vector<int> &buys) {
    GiftCard card(start);
    for (int b : buys) {
        card.pay(b);
    }
    return card.balance;
}