PAT 1050 String Subtraction

#include<iostream>
#include<string.h>//想用memset 在g++下要包这个头文件
#include<string>
using namespace std;
int main()
{
	string s1, s2,s3;
	getline(cin, s1);
	getline(cin, s2);
	char a[256];

	memset(a, -1, sizeof(a));
	for (int i = 0; i < s2.size(); ++i)
	{
		if (a[s2[i]] < 0)
		{
			a[s2[i]] = 1;
		}
	}
	for (int j = 0; j < s1.size(); j++)
	{
		if (a[s1[j]] < 0)
		{
			s3 += s1[j];
		}
	}
	cout << s3 << endl;
	return 0;
}