二叉树的遍历

二叉树的前序，中序，后序，层次遍历

/** 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);
    }
}