#define mian main
#define MAXN 10500 
using namespace std;
string a,b;
int na[MAXN], nb[MAXN], ans[MAXN]; 
bool pd;
int main(){
    cin >> a >> b;
	if((a < b && a.size() == b.size()) || a.size() < b.size()){
		swap(a, b);
		pd = true;
	}
    for(int i = a.size(); i > 0; i --)na[i] = a[a.size() - i] - '0';
    for(int i = b.size(); i > 0; i --)nb[i] = b[b.size() - i] - '0';
    int maxl = max(a.size(), b.size());
	for(int i = 1; i <= maxl; i ++)
	{
		if(na[i] < nb[i])
		{
			na[i + 1] --;
			na[i] += 10;
		}
		ans[i] = na[i] - nb[i];
	}
	
	while(ans[maxl] == 0)maxl --;
	
	if(pd == true)cout << "-"; 
	
	for(int i = maxl; i > 0; i --)cout << ans[i];
	if(maxl < 1)cout << "0";
    return 0;
}

来个精简的

#include<bits/stdc++.h>
using namespace std;
const int MAXN=10086+10;
char sa[MAXN],sb[MAXN],tmp[MAXN];
int a[MAXN],b[MAXN],c[MAXN];
int main(){
    cin>>sa>>sb;
    int la=strlen(sa);
    int lb=strlen(sb);
  
    if(la<lb || ( (la==lb) && strcmp(sa,sb)<0 ) ){
        strcpy(tmp,sa);
        strcpy(sa,sb);
        strcpy(sb,tmp);
        swap(la,lb);
        cout<<"-";
    }

    for(int i=0;i<la;i++) a[la-i]=sa[i]-'0';
    for(int i=0;i<lb;i++) b[lb-i]=sb[i]-'0';
    int lc=la>lb?la:lb;
    for(int i=1;i<=lc;i++) {
        if(a[i]<b[i]){
           a[i]+=10;
           a[i+1]--;
        }
        c[i]=a[i]-b[i];
    }

    while(c[lc]==0) lc--;

    for(int i=lc;i>=1;i--) cout<<c[i];

    return 0;
}

# include <bits/stdc++.h>
using namespace std;
string s1, s2, temp;
const int N = 1010;
int a[N], b[N], c[N];
void refine(string s, int a[]) {
	int len = s.size();
	int index = 0;
	for (int i = len - 1; i >= 0; i--) {
		a[index] = s[i] - '0';
		index++;
	}	
}
void sub(int a[], int b[], int  c[]) {
	for (int i = 0; i < N - 1; i++) {
		int num = a[i] - b[i];
		if (num < 0) {
			a[i + 1]--;
			num += 10;
		}
		c[i] = num;
	}
}
void print(int c[]) {
	int flag = false;
	for (int i = N - 1; i >= 0; i--) {
		if (c[i] != 0) {
			flag = true;
		}
		if (flag || i == 0) {
			cout << c[i];		
		}
	}	
}
bool cmp(string s1, string s2) {
	int len1 = s1.size();
	int len2 = s2.size();
	if (len1 != len2) {
		return len1 < len2;
	}
	for (int i = 0; i < len1; i++) {
		if (s1[i] != s2[i]) {
			return s1[i] < s2[i];
		}
	}
	return false;
}
int main() {
	cin >> s1 >> s2;
	if (cmp(s1, s2)) {
		temp = s1;
		s1 = s2;
		s2 = temp;
		printf("-");
	}
	refine(s1, a);
	refine(s2, b);
	sub(a, b, c);
	print(c);
	return 0; 
}

a = input()
b = input()
print(int(a) - int(b))