R 格式 参考代码

#include <bits/stdc++.h>
using namespace std;

typedef long long LL;
const int maxn = 2000 + 100;
struct BigInteger {
    int length;
    int num[maxn];

    void clear() {
        length = 0;
        memset(num, 0, sizeof(num));
    }

    BigInteger() {
        clear();
        length = 1;
    }

    BigInteger(int x) {
        clear();
        if (x == 0) {
            length = 1;
            return ;
        }
        while (x != 0) {
            num[length++] = x % 10;
            x /= 10;
        }
    }

    BigInteger(const string &str) {
        clear();
        for (int i = str.length() - 1; i >= 0; --i) {
            if (str[i] == '.') {
                continue;
            }
            num[length++] = str[i] - '0';
        }
        fixLength();
    }

    void fixLength() {
        for (int i = maxn - 1; i >= 0; --i) {
            if (num[i] != 0) {
                length = i + 1;
                return ;
            }
        }
        length = 1;
    }

    void fix() {
        int s = 0;
        for (int i = 0; i < maxn; ++i) {
            num[i] += s;
            s = num[i] / 10;
            num[i] %= 10;
        }
        fixLength();
    }
};

BigInteger operator+(const BigInteger &a, const BigInteger &b) {
    BigInteger c;
    int len = max(a.length, b.length);
    for (int i = 0; i < len; ++i) {
        c.num[i] = a.num[i] + b.num[i];
    }
    c.fix();
    return c;
}

BigInteger operator*(const BigInteger &a, const BigInteger &b) {
    BigInteger c;
    for (int i = 0; i < a.length; ++i) {
        for (int j = 0; j < b.length; ++j) {
            c.num[i + j] += a.num[i] * b.num[j];
        }
    }
    c.fix();
    return c;
}

ostream& operator<<(ostream &out, const BigInteger &x) {
    for (int i = x.length - 1; i >= 0; --i) {
        out << x.num[i];
    }
    return out;
}

BigInteger fastPow(BigInteger res, int n) {
    BigInteger ans;
    for (ans = 1; n != 0; n >>= 1) {
        if ((n & 1) == 1) {
            ans = ans * res;
        }
        res = res * res;
    }
    return ans;
}

int n, dig;
string d;

int main() {
#ifdef ExRoc
    freopen("test.txt", "r", stdin);
#endif
    ios::sync_with_stdio(false);

    cin >> n >> d;
    for (int i = d.length() - 1; i >= 0; --i) {
        if (d[i] == '.') {
            dig = d.length() - i - 1;
            break;
        }
    }
    BigInteger ans = fastPow(2, n) * BigInteger(d);
    if (ans.num[dig - 1] >= 5) {
        ans = ans + fastPow(10, dig);
    }
    for (int i = ans.length - 1; i >= dig; --i) {
        cout << ans.num[i];
    }
    cout << endl;

    return 0;
}