高精度 X 高精度

#include<bits/stdc++.h>
#define ULL unsigned long long
#define LL long long
#define PII pair<int,int>
using namespace std;
const int N = 3 * 1e6 + 10,M = 2 * 1e3 + 10,inf = 0x3f3f3f3f;

vector<int> sup_mul(vector<int> &a,vector<int> &b)
{
    vector<int> t(500005);
    for(int i=0;i<b.size();i++)
    {
        for(int j=0;j<a.size();j++)
        {
            t[i+j] += a[j]*b[i];
        }
    }
    for(int i=0;i<a.size()+b.size()-1;i++)
    {
        if(t[i] > 9)
        {
            t[i+1] += t[i]/10;
            t[i] %= 10;
        }
    }
    while(t.size()>1&&t.back()==0) t.pop_back();
    return t;
}
int main()
{
    ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    string a,b;
    cin>>a>>b;
    vector<int> A,B;
    for(int i=a.size()-1;i>=0;i--) A.push_back(a[i]-'0');
    for(int i=b.size()-1;i>=0;i--) B.push_back(b[i]-'0');

    auto c = sup_mul(A,B);

    for(int i=c.size()-1;i>=0;i--) cout<<c[i];
    return 0;
}

参考答案：

#include<iostream>
#include<vector>
#include<string>

using namespace std;

vector<int> mul(vector<int>& A, int b)
{
    vector<int>C;
    long long t = 0;

    for (int i = 0; i < A.size() || t; ++i)
    {
        if (i < A.size())t += A[i] * b;
        C.push_back(t % 10);
        t /= 10;
    }

    while (C.size() > 1 && C.back() == 0) C.pop_back();

    return C;
}
int main()
{
    int b;
    string a;
    vector<int > A;

    cin >> a >> b;

    for (int i = a.size() - 1; i >= 0; --i) A.push_back(a[i] - '0');

    vector<int>C = mul(A, b);

    for (int i = C.size() - 1; i >= 0; --i) cout << C[i];

    return 0;
}