二叉树的建立&&前中后遍历(递归实现)&&层次遍历

下面代码包含了二叉树的建立过程，以及三种遍历方法了递归实现，代码中还利用队列实现了层次遍历。

import java.util.LinkedList;
import java.util.Queue;

class TreeNode<K> {
    TreeNode<K> left, right;
    K key;
    public TreeNode(K k) {
        left = null; right = null; key = k;
    }
}

class BinaryTree<K extends Comparable<K> > { //添加 extends Comparable<K>后，K对象可以进行比较。
    TreeNode<K> root;
    public BinaryTree() {
        root = null;
    }
    public void createTree(TreeNode<K> node, K data) { //建立二叉树
        if(root == null) {
            root = new TreeNode<K>(data);
        } else {
            if(data.compareTo(node.key) < 0) {
                if(node.left == null)
                    node.left = new TreeNode<K>(data);
                else
                    createTree(node.left, data);
            } else {
                if(node.right == null)
                    node.right = new TreeNode<K>(data);
                else
                    createTree(node.right, data);
            }
        }
    }
    public void preOrder(TreeNode<K> root) { //前序遍历
        if(root != null) {
            System.out.println(root.key);
            preOrder(root.left);
            preOrder(root.right);
        }
    }
    public void inOrder(TreeNode<K> root) { //中序遍历
        if(root != null) {
            inOrder(root.left);
            System.out.println(root.key);
            inOrder(root.right);
        }
    }
    public void postOrder(TreeNode<K> root) { //后序遍历
        if(root != null) {
            postOrder(root.left);
            postOrder(root.right);
            System.out.println(root.key);
        }
    }
    public void levelOrder(TreeNode<K> root) { //层次遍历
        if(root != null) {
            Queue<TreeNode<K>> queue = new LinkedList<TreeNode<K>>();
            queue.offer(root);
            while (!queue.isEmpty()) {
                TreeNode<K> node = queue.poll();
                System.out.println(node.key);
                if(node.left != null)
                    queue.offer(node.left);
                if(node.right != null)
                    queue.offer(node.right);
            }
        }
    }
}
public class TestClass {
    public static void main(String[] args) {
        int[] array = {6, 8, 7, 5, 2, 5};
        BinaryTree<Integer> bt = new BinaryTree<Integer>();
        for (int i = 0; i < array.length; i++) {
            bt.createTree(bt.root, array[i]);
        }
//        bt.preOrder(bt.root);
//        bt.inOrder(bt.root);
//        bt.postOrder(bt.root);
        bt.levelOrder(bt.root);
    }
}