static boolean insertOneCharacter(String s1, String s2) {
		int i=0;
		for (;i<s1.length();++i)
			if (s1.charAt(i)!=s2.charAt(i)) break;
		for (;i<s1.length();++i)
			if (s1.charAt(i)!=s2.charAt(i+1)) return false;
		return true;
	}