string模拟

#include <iostream>
using namespace std;


string rev(string a){
    string res;
    for(char c:a) res = c + res;
    return res;
}

string remove_pre0(string a){
    int i = a.size() - 1;
    while(a.size() >= 1){
        if(a[i] == '0') a = a.substr(0, a.size()-1), i = a.size() - 1;
        else break;
    }
    if(a == "") return "0";
    return a;
}

// a >= b，返回true
bool ge(string a, string b){ 
    if(a.size() != b.size()) return a.size() > b.size();
    int len = a.size();
    for(int i = 0; i < len; i++){
        if(a[i] - '0' > b[i] - '0') return true;
        else if(a[i] - '0' < b[i] - '0') return false;
    }
    return true;
}

string add(string a, string b){
    int len = min(a.size(), b.size());
    string res;
    int carry = 0;
    int i = 0;
    for(i = 0; i < len; i++){
        int sum = a[i] - '0' + b[i] - '0' + carry;
        carry = sum / 10;
        sum %= 10;
        res += to_string(sum);
    }
    while(i < a.size()){
        int sum = a[i++] - '0' + carry;
        carry = sum / 10;
        sum %= 10;
        res += to_string(sum);       
    }
    while(i < b.size()){
        int sum = b[i++] - '0' + carry;
        carry = sum / 10;
        sum %= 10;
        res += to_string(sum);       
    }  
    if(carry) res += to_string(carry);
    return res;
}

string mul_helper(string a, string b){ // a x 单数字b
    string c;
    int len = a.size();
    int y = b[0] - '0';
    int carry = 0;
    int i = 0;
    for(i = 0; i < len; i++){
        int x = a[i] - '0';
        int pro = x * y + carry;
        carry = pro / 10; 
        pro %= 10;
        c += to_string(pro);
    }
    if(carry) c += to_string(carry);
    return c;
}


// 输入正序的a、b，返回反序的a*b
string mul(string a, string b){
    a = rev(a), b = rev(b);
    if(a.size() < b.size()) swap(a, b);
    string res;
    for(int i = 0; i < b.size(); i++){
        string pro = mul_helper(a, to_string(b[i] - '0'));
        int tmp = i;
        while(tmp--){
            pro = '0' + pro;
        }
        res = add(res, pro);
    }
    res = remove_pre0(res);
    return res;
}


int main(){
    string a, b;
    cin >> a >> b;
    cout << rev(mul(a, b));
}