本地跑没有问题，一提交全RE了

#include <bits/stdc++.h>
using namespace std;
struct BigInt{
    static const int BASE=1e8;
    static const int WIDTH=8;
    vector <int> s;

    BigInt (long long num=0) {*this=num;}
    BigInt operator = (long long num){
        s.clear();
        do{
            s.push_back(num%BASE);
            num/=BASE;
        } while(num>0);
        return *this;
    }
    BigInt operator = (const string& str){
        s.clear();
        int x,len=(str.length()-1)/WIDTH+1;
        for(int i=0;i<len;i++){
            int end=str.length()-i*WIDTH;
            int start=max(0,end-WIDTH);
            sscanf(str.substr(start,end-start).c_str(),"%d",&x);
            s.push_back(x);
        }
        return *this;
    }
    BigInt operator + (const BigInt& b) const {
        BigInt c;
        c.s.clear();
        for(int i=0,g=0;;i++)
        {
            if(g==0&&i>=s.size()&&i>=b.s.size()) break;
            int x=g;
            if(i<s.size()) x+=s[i];
            if(i<b.s.size()) x+=b.s[i];
            c.s.push_back(x%BASE);
            g=x/BASE;
        }
        return c;
    }
    BigInt operator += (const BigInt& b) {
        *this=*this+b; return *this;
    }
    BigInt operator * (const BigInt& b) const {
        BigInt c; c.s.clear(); long long n;
        for(int i=0;i<b.s.size()+s.size();i++) c.s.push_back(0);
        for(int i=0;i<b.s.size();i++)
            for(int j=0;j<s.size();j++) {
                n=((long long)b.s[i])*((long long)s[j]);
                c.s[i+j+1]+=(c.s[i+j]+(int)(n%BASE))/BASE;
                c.s[i+j]=(c.s[i+j]+(int)(n%BASE))%BASE;
                c.s[i+j+2]+=(c.s[i+j+1]+(int)(n/BASE))/BASE;
                c.s[i+j+1]=(c.s[i+j+1]+(int)(n/BASE))%BASE;
            }
        for(int i=c.s.size()-1;i>0;i--) if(c.s[i]==0) c.s.pop_back();
        return c;           
    }
    BigInt print(){
        for(int i=s.size()-1;i>=0;i--) printf("%d",s[i]);
    }

};
int main()
{
    string s1,s2;
    cin>>s1>>s2;
    BigInt a,b,c;
    a=s1;
    b=s2;
    c=a*b;

    c.print();

}