public static boolean isIsomorphic(String s1, String s2) {
		if (s1.length()!=s2.length()) return false;
		char[]map1=new char[256],map2=new char[256];
		Arrays.fill(map1, ' '); Arrays.fill(map2, ' ');
		for (int i=0;i<s1.length();++i) {
			char c1=s1.charAt(i),c2=s2.charAt(i);
			if (map1[c1]==' '&&map2[c2]==' ') { 
				map1[c1]=c2;map2[c2]=c1;
			} else return false;
		}
		return true;
	}