人见人爱A-B

#include <iostream>
#include <cstdio>
#include <set>
#include <algorithm>
using namespace std;

int main()
{
	int n, m, p[110], q[110], r[110];
	while(scanf("%d %d", &n, &m) && (n != 0 || m != 0))
	{
		set<int> a, b, c;
		
		for(int i = 0; i < n; ++ i)
		{
			scanf("%d", &p[i]);
			a.insert(p[i]);
		}
		for(int i = 0; i < m; ++ i)
		{
			scanf("%d", &q[i]);
			b.insert(q[i]);
		}
		
		int count = 0;
		for(int i = 0; i < n; ++ i)
		{
			if(b.find(p[i]) == b.end())
			{
				c.insert(p[i]);
				count ++;
			}
		}
		if(count == 0)	cout << "NULL";
		else
		{
			for(set<int>::iterator it = c.begin(); it != c.end(); ++ it)
			{
				cout << *it << " ";
			}
		}
		cout << endl;
	}
	
	return 0;
}

　　

注意: 每次都要清空集合!!!