C :

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
char a[1001],b[1001],t[1001],r[1001],tmp[1];
int x[1001],y[1001],z[1001];
int main(){
   scanf("%s%s",&a,&b);
   //先比较 
    if(strcmp(a,b)<0){
    	strcpy(t,a);
    	strcpy(a,b);
    	strcpy(b,t);
	}
	//把a字符串、b字符串分别放入两个整数数组中
	int i,alen =strlen(a),blen=strlen(b),n=alen>blen?alen:blen;
	for(i=0;i<alen;i++) x[i] = a[alen-i-1] - '0';
	for(i=0;i<blen;i++) y[i] = b[blen-i-1] - '0';
	for(i=0;i<n;i++){
		z[i] = x[i] - y[i];
		if(z[i]<0){
			z[i] = z[i]+10;
			x[i+1] = x[i+1] - 1;
		} 
	}
	int k;
	for(i=n-1;i>=0;i--){
		if(z[i]!=0){
			k = i;
			break;
		}
	}
	long long m = 0;
	for(i=k;i>=0;i--){
		m = m*10 + z[i];
	}
	if(m==0){
	 	printf("0");
		return 0; 
	} 
	while(m!=0){
		if(m%16>=10){
			tmp[0] = m%16-10+'A';
			strcat(r,tmp);
		}else{
			tmp[0] = m%16+'0';
			strcat(r,tmp);
		}
		m = m/16;
	}
	for(i=strlen(r)-1;i>=0;i--) printf("%c",r[i]);
    return 0;
}

C++ :

#include <bits/stdc++.h>
using namespace std;

#include <bits/stdc++.h>
using namespace std;

//高精度减法之后转long long 
long long fun(string s1,string s2) {
	long long a[250] = {0},b[250] = {0},c[250] = {0};
	long long i,len,p = 0,r = 0;
	//长的一定大，一样长字典码大的一定大
	//"123" "3"   "123" "125"
	if(s1.size() < s2.size() || (s1.size() == s2.size() && s1 < s2)) {
		swap(s1,s2);//直接交换两个变量的值
	}

	//将s1和s2逆序存入整数数组
	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';
	}

	//逐位相减
	len = s1.size();

	for(i = 0; i < len; i++) {
		//如果不够减，向右借1，当10用
		if(a[i] < b[i]) {
			a[i + 1] = a[i + 1] - 1;
			a[i] = a[i] + 10;
		}

		c[i] = a[i] - b[i];
	}

	//从右向左逐位输出，从第一个遇到的非0元素开始输出
	for(i = len - 1; i >= 0; i--) {
		if(c[i] != 0) {
			p = i;
			break;
		}
	}

	//逆序从第一个非0元素 输出每一位
	//123 -> 321
	long long t = 1;
	for(i = 0; i <= p; i++) {
		r = r + t * c[i];
		t = t * 10;
	}
	
	return r;
}

//转16进制 
string jinzhi(long long n){
	string t = "0123456789ABCDEF";
	string r = "";
	
	if(n == 0) r = "0";
	while(n != 0){
		r = t[n%16] + r;
		n = n / 16;
	}
	
	return r;
}

int main() {
	string s1,s2;
	cin>>s1>>s2;
	cout<<jinzhi(fun(s1,s2));
}