二叉树的建立&&前中后遍历(递归实现)&&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);
}
}