#include <bits/stdc++.h>
using namespace std;
bool cmp(vector<int> &A, vector<int> &B) //判断是否有 A>=B
{
    if (A.size() != B.size())
        return A.size() > B.size();
    for (int i = A.size() - 1; i >= 0; i--) //比较首位
    {
        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; //借位
    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); //二合一 t>=0 t=t：不需要借位；t<0 t=t+10：需要借位
        if (t < 0)
            t = 1;
        else
            t = 0;
    }
    //去除高位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); //保证 A >= B;
    else
        C = sub(B, A),cout << '-';
    for (int i = C.size() - 1; i >= 0; i--)
        cout << C[i];
    return 0;
}

求求了，点个赞吧！

#include <bits/stdc++.h>
using namespace std;
string s1,s2;
int a[11000000],b[11000000],c[11000000],len,p;
int i;
char f='+';
int main(){
	cin>>s1>>s2;
	if(s1.size()<s2.size()||(s1.size()==s2.size()&&s1<s2)){
		f='-';
		swap(s1,s2);
	}
	for(i=0;i<s1.size();i++){
		a[i]=s1[s1.size()-i-1]-'0';
	}
	for(i=0;i<s2.size();i++){
		b[i]=s2[s2.size()-i-1]-'0';
	}
	len=s1.size();
	for(i=0;i<len;i++){
		if(a[i]<b[i]){
			a[i+1]=a[i+1]-1;
			a[i]=a[i]+10;
		}
		c[i]=a[i]-b[i];
	}
	if(f=='-') cout<<f;
	for(i=len-1;i>=0;i--){
		if(c[i]!=0){
			p=i;
			break;
		}
	}
	for(i=p;i>=0;i--){
		cout<<c[i];
	}
	return 0;
}