/** binary tree traversal methods */package dataStructures;public class BinaryTreeTraversal {    /** visit method that prints the element in the node */    public static void visit(BinaryTreeNode t) {        System.out.print(t.element + " ");    }    /** preorder traversal */    public static void preOrder(BinaryTreeNode t) {        if (t != null) {            visit(t); // visit tree root            preOrder(t.leftChild); // do left subtree            preOrder(t.rightChild); // do right subtree        }    }    /** inorder traversal */    public static void inOrder(BinaryTreeNode t) {        if (t != null) {            inOrder(t.leftChild); // do left subtree            visit(t); // visit tree root            inOrder(t.rightChild); // do right subtree        }    }    /** postorder traversal */    public static void postOrder(BinaryTreeNode t) {        if (t != null) {            postOrder(t.leftChild); // do left subtree            postOrder(t.rightChild); // do right subtree            visit(t); // visit tree root        }    }    /** level order traversal      *  借助队列实现层次遍历     * */    public static void levelOrder(BinaryTreeNode t) {        ArrayQueue q = new ArrayQueue();        while (t != null) {            visit(t); // visit t            // put t‘s children on queue            if (t.leftChild != null)                q.put(t.leftChild);            if (t.rightChild != null)                q.put(t.rightChild);            // get next node to visit            t = (BinaryTreeNode) q.remove();        }    }    /** test program */    public static void main(String[] args) {        BinaryTreeNode y = new BinaryTreeNode(new Integer(2));        BinaryTreeNode z = new BinaryTreeNode(new Integer(3));        BinaryTreeNode x = new BinaryTreeNode(new Integer(1), y, z);        BinaryTreeNode m = new BinaryTreeNode(new Integer(5));        BinaryTreeNode n = new BinaryTreeNode(new Integer(0), x, m);        System.out.println("The elements in preorder are");        preOrder(n);        System.out.println();        System.out.println("The elements in inorder are");        inOrder(n);        System.out.println();        System.out.println("The elements in postorder are");        postOrder(n);        System.out.println();        System.out.println("The elements in level order are");        levelOrder(n);    }}