public static List<String> generateNGramList(List<String> sentence, int n){
		List<String> nGramSentence = new ArrayList<String>(sentence.size());
		if (sentence.size() < n){
			return nGramSentence;
		}
		for (int i = 0; i < sentence.size()-n+1; i++){
			StringBuilder sb = new StringBuilder();
			for (int j = 0; j < n; j++){
				sb.append(sentence.get(i+j).toLowerCase());
				sb.append("+");
			}
			nGramSentence.add(sb.toString());
		}
		return nGramSentence;
	}