15RE求助！

#include <bits/stdc++.h>
#define IOS ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define endl "\n"
#define itn int
#define nit int
#define nti int
#define tin int
#define tni int
#define int long long
using namespace std;
string a, b;
vector<int> vec;
string ans(string A, string B){
    int a[1005] = {}, b[1005] = {}, c[1005] = {};
    for(int i = A.size() - 1, j = 1; i >= 0; i--, j++) a[j] = A[i] - '0';
    for(int i = B.size() - 1, j = 1; i >= 0; i--, j++) b[j] = B[i] - '0';
    int len = max(A.size(), B.size());
    for(int i = 1; i <= len; ++i){
        c[i] += a[i] - b[i];
        if(c[i] < 0) c[i + 1] --, c[i] += 10;
    }
    string C;
    for(int i = len; i >= 1; i--) {
        C += char(c[i] + '0');
    }
    int pos = -1;
    for(int i = 0; i < C.size(); ++i) {
        if(C[i] != '0'){
            pos = i;
            break;
        }
    }
    if(pos == -1) return "0";;;;;;;;;;;;;;
    return C.substr(pos);
}
signed main(){
	IOS
	cin >> a >> b;
	cout << ans(a, b);
	return 0;
}