参考答案：

#include<iostream>
#include<vector>
#include<string>
#include<algorithm>

using namespace std;

vector<int>A, B, C;

bool cmp(vector<int>& A, vector<int>& B)
{
    if (A.size() != B.size()) return A.size() > B.size();

    for (int i = A.size() - 1; i >= 0; --i)
    {
        if (A[i] != B[i]) return A[i] > B[i];
    }

    return true;
}
vector<int>sub(vector<int>& A, vector<int>& B)
{
    vector<int> C;
    int t = 0;

    for (int i = 0; i < A.size(); ++i)
    {
        t = A[i] - t;
        if (i < B.size()) t -= B[i];
        C.push_back((t + 10) % 10);
        if (t < 0) t = 1;
        else t = 0;
    }

    while (C.size() > 1 && C.back() == 0) C.pop_back();

    return C;
}
int main()
{
    string a, b;

    cin >> a >> b;

    for (int i = a.size() - 1; i >= 0; --i) A.push_back(a[i] - '0');
    for (int i = b.size() - 1; i >= 0; --i) B.push_back(b[i] - '0');

    if (cmp(A, B)) C = sub(A, B);
    else C = sub(B, A), cout << '-';

    for (int i = C.size() - 1; i >= 0; --i) cout << C[i];

    return 0;
}