高精度减法

给定两个正整数，计算它们的差。

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#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; }