public String mapMerge(String s1, String s2, String s3) {
		boolean[]f=new boolean[256];
		for (char c:s1.toCharArray()) f[c]=true;
		for (char c:s2.toCharArray()) f[c]=true;
		for (char c:s3.toCharArray()) f[c]=true;
		StringBuilder s=new StringBuilder();
		for (int i=0;i<256;++i) if (f[i])s.append((char)i);
		return s.toString();
	}