String longestPrefix(Node c, int i, String s, StringBuilder prefix) {
			if (i==s.length()) 
				return prefix.toString();
			int id=s.charAt(i)-'a';
			if (c.child[id]!=null) {
				return longestPrefix(c.child[id], i+1, s, prefix.append(s.charAt(i)));
			}
			return prefix.toString();
		}