class Tree<E>{

    E element;

    Tree<E> lChild;

    Tree<E> rChild;

    public Tree(E e){

        element = e;

    }

}

public class BinaryTree {

    /**

     * 树形如下：

     *         1

     *     / \

     *    2   3

     *    \   / \

     *     4  5  6

     */

    public static void main(String[] args) {

        Tree<Integer> n1 = new Tree<Integer>(1);

        Tree<Integer> n2 = new Tree<Integer>(2);

        Tree<Integer> n3 = new Tree<Integer>(3);

        Tree<Integer> n4 = new Tree<Integer>(4);

        Tree<Integer> n5 = new Tree<Integer>(5);

        Tree<Integer> n6 = new Tree<Integer>(6);

        System.out.println("Construct the tree...");

        n2.rChild=n4;

        n3.lChild=n5;

        n3.rChild=n6;

        n1.lChild=n2;

        n1.rChild=n3;

        System.out.println("打印先序遍历结果：");

        firstOrder(n1);

        System.out.println("\n打印中序遍历结果：");

        midOrder(n1);

        System.out.println("\n打印后序遍历结果：");

        lastOrder(n1);

    }

    public static <E> void firstOrder(Tree<E> root){

        if(root!=null){

            System.out.print(root.element+" ");

            firstOrder(root.lChild);

            firstOrder(root.rChild);

        }

    }

    public static <E> void lastOrder(Tree<E> root){

        if(root!=null){

            lastOrder(root.lChild);

            lastOrder(root.rChild);

            System.out.print(root.element+" ");

        }

    }

    public static <E> void midOrder(Tree<E> root){

        if(root!=null){

            midOrder(root.lChild);

            System.out.print(root.element+" ");

            midOrder(root.rChild);

        }

    }

}

Construct the tree...

打印先序遍历结果：

1 2 4 3 5 6

打印中序遍历结果：

2 4 1 5 3 6

打印后序遍历结果：

4 2 5 6 3 1