前方高能 100% AC

---

#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <cwchar>
#include <cwctype>
#if __cplusplus >= 201103L
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdalign>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cuchar>
#endif
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <chrono>
#include <codecvt>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#endif
#if __cplusplus >= 201402L
#include <shared_mutex>
#endif
#if __cplusplus >= 201703L
#include <any>
#include <charconv>
#include <filesystem>
#include <optional>
#include <memory_resource>
#include <string_view>
#include <variant>
#endif
#if __cplusplus > 201703L
#include <barrier>
#include <bit>
#include <compare>
#include <concepts>
#if __cpp_impl_coroutine
# include <coroutine>
#endif
#include <latch>
#include <numbers>
#include <ranges>
#include <span>
#include <stop_token>
#include <semaphore>
#include <source_location>
#include <syncstream>
#include <version>
#endif
using namespace std;
#define N 505
void Multiply(int a[], int b[], int r[])
{
	int i;
    for(i = 1; i <= a[0]; ++i)
    {
        int c = 0;
        for(int j = 1; j <= b[0]; ++j)
        {
            r[i+j-1] += a[i]*b[j] + c;
            c = r[i+j-1] / 10;
            r[i+j-1] %= 10;
        }
        r[i+b[0]] += c;
    }
    i = a[0] + b[0];
    while(r[i] == 0 && i > 1)
        i--;
    r[0] = i;
}
void toNum(char s[], int a[])
{
    a[0] = strlen(s);
	for(int i = 1; i <= a[0]; ++i)
        a[i] = s[a[0] - i] - '0';
}
void showNum(int a[])
{
    for(int i = a[0]; i >= 1; --i)
        cout << a[i];
}
int main()
{
    int a[N] = {}, b[N] = {}, r[N] = {};
    char s[N];
    cin >> s;
    toNum(s, a);
    cin >> s;
    toNum(s, b);
    Multiply(a, b, r);
    showNum(r);
    return 0;
}

---

直接秒了