string字符实现

#include <iostream>
using namespace std;
string a, b, rev_a, rev_b;

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

// 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 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;
}

string sub(){
    string res;
    int neg = 0;
    if(!ge(a, b) && a != b) swap(rev_a, rev_b), neg = 1; // a < b，返回结果加上负号
    int len = rev_b.size();
    int borrow = 0;
    int i = 0;
    for(; i < len; i++){
        int x = rev_a[i] - '0', y = rev_b[i] - '0';
        if(borrow) x -= borrow, borrow = 0;
        if(x < y){
            borrow++;
            x += 10;
        }
        res += to_string(x - y);
    }

    int len_a = rev_a.size();
    while(i < len_a){
        int x = rev_a[i] - '0';
        if(borrow) x -= borrow, borrow = 0;
        if(x < 0){
            borrow++;
            x += 10;
        }
        res += to_string(x);
        i++;
    }
    res = remove_pre0(res);
    if(neg) res += "-";
    return res;
}

int main(){
    cin >> a >> b;
    rev_a = rev(a);
    rev_b = rev(b);
    cout << rev(sub());
}