高精度减法

给定两个正整数，计算它们的差。

#include <iostream>
#include <vector>
using namespace std;
vector<int> a, b, c;

bool cmp () {
    if (a.size() != b.size()) return a.size() > b.size();
    for (int i = a.size() - 1; i >= 0; i--) {
        if (a[i] == b[i]) continue;
        return a[i] > b[i];
    }
    return true;
}

void sub (vector<int>& a, vector<int>& b) {
    for (int i = 0, t = 0; i < a.size(); i++) {
        t = a[i] - t;
        if (i < b.size()) t -= b[i];
        
        if (t >= 0) {
            c.push_back(t);
            t = 0;
        }
        else {
            t += 10;
            c.push_back(t);
            t = 1;
        }
    }
    
    while (c.size() > 1 && c.back() == 0) c.pop_back();
}

int main() {
    string x, y;
    cin >> x >> y;
    
    for (int i = x.size() - 1; i >= 0; i--) a.push_back(x[i] - '0');
    for (int i = y.size() - 1; i >= 0; i--) b.push_back(y[i] - '0');
    
    if (cmp()) sub(a, b);
    else {
        cout << "-";
        sub(b, a);
    }
    
    for (int i = c.size() - 1; i >= 0; i--) cout << c[i];
    return 0;
}