#include<bits/stdc++.h>
using namespace std;
 
string s1,s2;
int a[250],b[250],c[250];
char f = '+';
int len,p;
int main(){
	freopen("sub.in","r",stdin);
	freopen("sub.out","w",stdout);
	cin >> s1 >> s2;
	if (s2.size() > s1.size() || (s1.size() == s2.size() && s2 > s1)) {
		swap(s1,s2);
		f = '-';
	}
	
	for (int i = 0; i < s1.size(); i++) {
		a[s1.size() - i - 1] = s1[i] - '0';
	}
	for (int i = 0; i < s2.size(); i++) {
		b[s2.size() - i - 1] = s2[i] - '0';
	}
	
	len = s1.size();
	for (int i = 0; i < len; i++) {
		if (a[i] < b[i]) {
			a[i + 1] -= 1;
			a[i] += 10;
		}
		c[i] = a[i] - b[i];
	}
	if (f == '-') cout << f;
	
	for (int i = len - 1; i >= 0; i--) {
		if (c[i] != 0) {
			p = i;
			break;
		}
	}
	
	for (int i = p; i >= 0; i--) cout << c[i];
}

这道题用三个数组就行。

第一步：把输入的字符串转换成数组（因为都是数字）

第二步：相减，不够就借位

第三步：判断是否是负数

第四步：输出