@Override
	public Sentence_tree find_common_node(Sentence_tree keyword_tree){
		Sentence_tree temp1 = this;
		LinkedList<Sentence_tree> tree_list1 = new LinkedList<Sentence_tree>();
		Sentence_tree temp2 = keyword_tree;
		LinkedList<Sentence_tree> tree_list2 = new LinkedList<Sentence_tree>();
		while(temp1 != null){
			tree_list1.addLast(temp1);
			temp1 = temp1.get_parent();
		}
		while(temp2 != null){
			tree_list2.addLast(temp2);
			temp2 = temp2.get_parent();
		}
		for(int index1 = 0; index1 < tree_list1.size(); index1++){
			for(int index2 = 0; index2 < tree_list2.size(); index2++){
				if(tree_list1.get(index1) == tree_list2.get(index2)){
					return tree_list2.get(index2);
				}
			}
		}
		return null;
	}