#include<iostream>
#include<string>
#include<vector>
using namespace std;

string a,b;
vector<int> A,B,C;

bool cmp(vector<int> &A, vector<int> &B)
{
    if(A.size() != B.size())
        return A.size() > B.size();
    else
    {
        for(int i = A.size()-1; i >= 0; i --)
        {
            if(A[i] != B[i])
                return A[i] > B[i];
        }
    }
    return true;//A和B一模一样
}
vector<int> sub(vector<int> &A, vector<int> &B)//默认A>=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];

        if(t >= 0)
        {
            C.push_back(t);
            t = 0;
        }
        else
        {
            C.push_back(t+10);
            t = 1;
        }
    }
    while(C.size() > 1 && C.back() == 0)
        C.pop_back();
    return C;
}

int main()
{
    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;
}

1. 注意for(int i = C.size()-1; i >= 0; i --)-1!!
2. vector<int> A,B,C;全局变量定义之后别重复定义了！！