void postOrder(Node root) {
    Node t = root;
    Deque<Node> stack = new ArrayDeque<Node>();
    stack.push(root);
    while(!stack.isEmpty() && root!=null){
        root = stack.peek();
        //nodes without children should be printed 
        if( (root.left==null&&root.right==null) 
         || (t==root.left||t==root.right) ){//or nodes whose children have already been printed 
            System.out.print(root.data+" ");
            stack.pop();
            t = root;
        }else{
            if(root.right!=null) stack.push(root.right);
            if(root.left!=null) stack.push(root.left);
        }
    }
}