大数模板（未完成）

string add ( string a,string b )
{
    int ans[1000] = { 0 };
    string str;
    reverse ( all(a) );
    reverse ( all(b) );
    int len = max ( a.size(),b.size() );
    if ( len == a.size() ){
        b.resize( len,'0' );
    }
    else{
        a.resize( len,'0' );
    }
    int tmp = 0;
    for ( int i = 0; i < len; i ++ ){
        ans[i] = a[i]- '0' + b[i] - '0' + tmp;
        tmp = ans[i] / 10;
        ans[i] %= 10;
    }
    if ( tmp != 0 ){
        ans[len] = tmp;
        len += 1;
    }
    str.resize( len,'0' );
    int cnt = 0;
    for ( int i = len - 1; i >= 0; i -- ){
        str[cnt++] = ans[i] + '0';
    }
    return str;
}

string muti ( string a,int b )
{
    int ans[1000] = { 0 };
    reverse ( all(a) );
    int len = a.size();
    int tmp = 0;
    for ( int i = 0; i < len; i ++ ){
        ans[i] = a[i] - '0';
        ans[i] = ans[i] * b + tmp;
        tmp = ans[i] / 10;
        ans[i] %= 10;
    }
    string str;
    if ( tmp != 0 ){
        ans[len] = tmp;
        len += 1;
    }
    str.resize( len,'0' );
    int cnt = 0;
    for ( int i = len - 1; i >= 0; i -- ){
        str[cnt++] += ans[i];// + '0';
    }
    return str;
}