public class Tree {

        // Privided, DON'T CHANGE
        // the root of the tree
	private TreeNode root;

	//empty tree
	public Tree(){
		this.root = null;
	}

	// rootItem, empty children
	public Tree(TreeNode root){
		this.root = root;
	}

	public boolean isEmpty(){
		return root==null;
	}

	public void preorderTraverse(){
		if (!isEmpty())
			preorderTraverse(root,"");
		else
			System.out.println("null");
	}

	// print root item
	// print left tree
	// print right tree
	public void preorderTraverse(TreeNode node, String indent){
		System.out.println(indent+node.getItem());
		if(node.getLeft()!=null) preorderTraverse(node.getLeft(),indent+" ");
		if(node.getRight()!=null) preorderTraverse(node.getRight(),indent+" ");

	}

	// if tree empty return null
	// else evaluate the tree by postorder traversal 
	// and return its value
	public Boolean postorderEval(){
		Boolean res = null;
		if(!isEmpty()){
			res = postorderEval(root);
		}
		return res;
	}


        // IMPLEMENT
    
	public Boolean postorderEval(TreeNode node){
	// evaluate left tree
	// evaluate right tree (if not null)
	// evaluate operator in node and return Boolean result
	return false;
	}	


        // EXERCISE
	public static void main(String[] args){
		System.out.println("Stepwise build infix expression: not not (true or false) and true");
		
		Tree t = new Tree();
		System.out.println("tree: "); t.preorderTraverse();	
		System.out.println("result:\n" + t.postorderEval()+"\n");
		
		TreeNode a = new TreeNode("true");
		t = new Tree(a);
		System.out.println("tree: "); t.preorderTraverse();	
		System.out.println("result:\n" + t.postorderEval()+"\n");
				
		TreeNode b = new TreeNode("false");
		TreeNode or = new TreeNode("or", a, b);
		t = new Tree(or);
		System.out.println("tree: "); t.preorderTraverse();	
		System.out.println("result:\n" + t.postorderEval()+"\n");
		
		TreeNode not2 = new TreeNode("not", or);
		t = new Tree(not2);
		System.out.println("tree: "); t.preorderTraverse();	
		System.out.println("result:\n" + t.postorderEval()+"\n");
		
		TreeNode not = new TreeNode("not", not2);
		TreeNode and = new TreeNode("and", not, new TreeNode("true"));
		t = new Tree(and);
		System.out.println("tree: "); t.preorderTraverse();	
		System.out.println("result:\n" + t.postorderEval()+"\n");
	}
	
}