转任意进制

string decimalToBase(int n, int base) {
    if (n == 0) return "0";
    const string digits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    string result = "";
    while (n > 0) {
        result = digits[n % base] + result;
        n /= base;
    }
    return result;
}

组合数

long long binomialCoefficient(int n, int k) {
    if (k < 0 || k > n) return 0;
    if (k > n - k) k = n - k; // 使用对称性优化
    long long c = 1;
    for (int i = 0; i < k; i++) {
        c = c * (n - i) / (i + 1);
    }
    return c;
}

通过stringstream实现string与int之间的类型转换

void example() {
    string res = "123 456";
    stringstream ss(res);
    string a;
    int b;
    ss >> a >> b;
}

文件操作

void save(string filename, const string& s) {
    ofstream out;
    out.open(filename, ios::binary | ios::app);
    if (out.is_open()) {
        out << s << endl;
    }
    out.close();
}

void read(string filename) {
    ifstream in;
    in.open(filename, ios::binary);
    if (in.is_open()) {
        string res;
        while(getline(in, res)) {
            stringstream ss;
            ss << res;
            string a, b;
            getline(ss, a, ' '), getline(ss, b);
            cout << a << ' ' << b << endl;
        }
    }
    in.close();
}

void file()
{
    string filename = "test.txt";
    save(filename, "Hello World!");
    save(filename, "Hello C++!");
    read(filename);
}