方法一

#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
int compare(string a, string b){
    if(a.size() > b.size())     return 1;
    else if(a.size() < b.size())    return -1;
    else{
        for(int i = 0; i < a.size(); i ++ ){
            if(a[i] > b[i])     return 1;
            else if(a[i] < b[i])    return -1;
        }   

    }
    return 1;
}
string a, b;
int main(){
    cin >> a >> b;
    int flag = compare(a, b);
    reverse(a.begin(), a.end());
    reverse(b.begin(), b.end());
    if(flag == -1){
        cout << "-";
        swap(a, b);
    }
    string res = "";
    int cnt = 0;
    int i;
    for(i = 0; i < a.size() && i < b.size(); i ++ ){
        int num = a[i] - b[i] - cnt;
        if(num < 0){
            num += 10;
            cnt = 1;
        }else   cnt = 0;
        res += (num + '0');
    }
    while(i < a.size()){
        int num = a[i] - '0' - cnt;
        if(num < 0){
            num += 10;
            cnt = 1;
        }else   cnt = 0;
        res += (num + '0');
        i ++ ;
    }
    reverse(res.begin(), res.end());
    flag = 1;
    for(int i = 0; i < res.size(); i ++ ){
        if(flag == 1 && res[i] == '0'){
            ;
        }else{
            cout << res[i];
            flag = 0;
        }   
    }
    if(flag == 1)   cout << 0 << endl;
    return 0;
}

方法二（精简版）

#include <iostream>
#include <vector>
using namespace std;
bool cmp(string a, string b){
    if(a.size() != b.size())    return a.size() > b.size();
    for(int i = 0; i < a.size(); 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;
    vector<int> 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');
    vector<int> C;
    if(cmp(a, b))   C = sub(A, B);
    else{
        cout << "-";
        C = sub(B, A);
    }
    for(int i = C.size() - 1; i >= 0; i -- )    cout << C[i];
    return 0;
}