二叉树的实现

package com.jokin.learn.Jdk18;

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

/**
 * 二叉树的创建及遍历
 * https://blog.youkuaiyun.com/qq_26312651/article/details/78144966
 */
public class MyBinTree {
    private Node root;

    public MyBinTree() {
        this.root = null;
    }
//java的访问权限有下面四种：public--都可访问(公有)   protected--包内和子类可访问（保护）不写(default)--包内可访问 （默认）private--类内可访问（私有）

    private class Node {
        int data;         //default修饰符
        Node lchild;
        Node rchild;

        public Node(int data) {
            this.data = data;
            this.lchild = null;
            this.rchild = null;
        }
    }


    /**
     *  往二叉树node 中插入数据data
     * @param node
     * @param data
     */
    //创建二叉查找树：所有节点的左子树节点都比根节点小，右子树节点都比根节点大
    public void buildTree(Node node, int data) {
        if (node == null) {
            node = new Node(data);
        /*  if(node==null){      开始没注意写成了这样，发现每次调用buildTree时，node都是null，发现底下传入的参数其实是root
            node= new LinkNode(data);*/
        } else {
            if (data < node.data) {  //左孩子
                if (node.lchild == null) {
                    node.lchild = new Node(data);
                } else {
                    buildTree(node.lchild, data);
                }
            } else {
                if (node.rchild == null) {
                    node.rchild = new Node(data);
                } else {
                    buildTree(node.rchild, data);
                }

            }
        }
    }


    //递归前序遍历   根----左---右
    public void preOrder(Node node) {
        if (node != null) {
            System.out.print("  " + node.data);
            preOrder(node.lchild);
            preOrder(node.rchild);
        }
    }

    //递归中序遍历
    public void inOrder(Node node) {
        if (node != null) {
            inOrder(node.lchild);
            System.out.print(" " + node.data);
            inOrder(node.rchild);
        }
    }

    //递归后序遍历
    public void postOrder(Node node) {
        if (node != null) {
            postOrder(node.lchild);
            postOrder(node.rchild);
            System.out.print(" " + node.data);
        }
    }

    //非递归前序遍历
    public void preOrder2(Node node) {
        Stack<Node> stack = new Stack<>();
        while (node != null || !stack.isEmpty()) {
            //先一路向左走，找到底，把找到的节点都压入栈，并sysout出来。
            while (node != null) {
                System.out.print(node.data + " ");
                stack.push(node);
                node = node.lchild;
            }
            if (!stack.isEmpty()) {  //抛出栈顶元素
                node = stack.pop();
                node = node.rchild;
            }
        }
    }

    //非递归的中序遍历
//和前序遍历的区别就是将输出放在了栈的pop之后
    public void inOrder2(Node node) {
        Stack<Node> stack = new Stack<>();
        while (node != null || !stack.isEmpty()) {
            while (node != null) {
                stack.push(node);
                node = node.lchild;
            }
            if (!stack.isEmpty()) {
                node = stack.pop();
                System.out.print(node.data + " ");
                node = node.rchild;
            }

        }
    }

    //后续遍历的非递归实现
    public void postOrder2(Node node) {
        Stack<Node> stack = new Stack<>();
        Node curNode = node;
        Node lastNode = node;
        while (curNode != null) {
            stack.push(curNode);
            curNode = curNode.lchild;
        }
        while (!stack.isEmpty()) {
            curNode = stack.pop();
            if (curNode.rchild == null || curNode.rchild == lastNode) {
                System.out.print(curNode.data + " ");
                lastNode = curNode;
            } else {
                stack.push(curNode);
                curNode = curNode.rchild;
                while (curNode != null) {
                    stack.push(curNode);
                    curNode = curNode.lchild;
                }
            }
        }

    }

    //层次遍历
    public void levelTravel(Node node) {
        Queue<Node> queue = new LinkedList<>();
        queue.offer(node);
        while (!queue.isEmpty()) {
            node = queue.poll();
            System.out.print(node.data + " ");
            if (node.lchild != null) {
                queue.offer(node.lchild);
            }
            if (node.rchild != null) {
                queue.offer(node.rchild);
            }
        }

    }


    public static void main(String[] args) {
        int[] a = {2, 4, 12, 45, 21, 6, 111};
        MyBinTree myBinTree = new MyBinTree();
        for (int i = 0; i < a.length; i++) {
            myBinTree.buildTree(myBinTree.root, a[i]);
        }
        myBinTree.preOrder(myBinTree.root);
        System.out.println();
        myBinTree.preOrder2(myBinTree.root);
        System.out.println();
        myBinTree.inOrder(myBinTree.root);
        System.out.println();
        myBinTree.inOrder2(myBinTree.root);
        System.out.println();
        myBinTree.postOrder(myBinTree.root);
        System.out.println();
        myBinTree.postOrder2(myBinTree.root);
        System.out.println();
        myBinTree.levelTravel(myBinTree.root);

    }
}