String longestPalindrome (String s){
		String longest;
		
		int l = s.length();
		if (l==0) return "";
		
		longest = s.substring(0,1); // a single char itself is a palindrome
		for (int i = 0; i < l-1; i++) {
			String s1 = expandCenter(s,i,i);
			if (s1.length() > longest.length()) longest = s1;
			
			String s2 = expandCenter(s,i,i+1);
			if (s2.length() > longest.length()) longest = s2;
		}
		
		return longest;
	}