二叉树递归及非递归遍历

package com.company;

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

/**
 * @author wuzhijun <wuzhijun@kuaishou.com>
 * Created on 2022-01-13
 */
public class TreeNodeOrder {
    public static void main(String[] args) {
        //        https://www.cnblogs.com/liuyang0/p/6271331.html

        System.out.println("前序遍历");
        preOrderRecur(buildTree());
        preOrder(buildTree());
        System.out.println("中序遍历");
        midOrderRecur(buildTree());
        midOrder(buildTree());
        System.out.println("后续遍历");
        postOrderRecur(buildTree());
        posOrder(buildTree());
        System.out.println("层级遍历");
        levelOrderRecur(buildTree());
        levelOrder(buildTree());
    }

    private static TreeNode buildTree() {

        List<TreeNode> nodes = new ArrayList<>();
        for (int i = 0; i <= 10; i++) {
            nodes.add(new TreeNode(i));
        }
        nodes.get(4).setLeft(nodes.get(2));
        nodes.get(4).setRight(nodes.get(6));
        nodes.get(2).setLeft(nodes.get(1));
        nodes.get(2).setRight(nodes.get(3));
        nodes.get(6).setLeft(nodes.get(5));
        nodes.get(6).setRight(nodes.get(7));
        nodes.get(7).setRight(nodes.get(8));
        nodes.get(8).setRight(nodes.get(10));
        return nodes.get(4);

    }

    //递归前序遍历
    private static void preOrderRecur(TreeNode node) {
        if (node == null) {
            return;
        }
        System.out.println(node.value);
        preOrderRecur(node.left);
        preOrderRecur(node.right);
    }

    //递归中序遍历
    private static void midOrderRecur(TreeNode node) {
        if (node == null) {
            return;
        }
        midOrderRecur(node.left);
        System.out.println(node.value);
        midOrderRecur(node.right);
    }

    //递归后序遍历
    private static void postOrderRecur(TreeNode node) {
        if (node == null) {
            return;
        }
        postOrderRecur(node.left);
        postOrderRecur(node.right);
        System.out.println(node.value);
    }

    //层序遍历
    private static void levelOrderRecur(TreeNode node) {
        if (node == null) {
            return;
        }
        int depth = depth(node);
        for (int i = 1; i <= depth; i++) {
            levelOrderRecur(node, i);
        }
    }

    private static void levelOrderRecur(TreeNode node, int level) {
        if (node == null || level < 1) {
            return;
        }
        if (level == 1) {
            System.out.println(node.value);
        }
        levelOrderRecur(node.left, level - 1);
        levelOrderRecur(node.right, level - 1);
    }

    private static int depth(TreeNode node) {
        if (node == null) {
            return 0;
        }
        int leftDepth = depth(node.left);
        int rightDepth = depth(node.right);
        return Math.max(leftDepth, rightDepth) + 1;
    }


    //非递归先序遍历
    private static void preOrder(TreeNode node) {
        if (node == null) {
            return;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(node);
        while (!stack.isEmpty()) {
            TreeNode item = stack.pop();
            System.out.println(item.getValue());
            if (item.getRight() != null) {
                stack.push(item.getRight());
            }
            if (item.getLeft() != null) {
                stack.push(item.getLeft());
            }
        }
    }

    //非递归中序遍历
    private static void midOrder(TreeNode node) {
        if (node == null) {
            return;
        }
        Stack<TreeNode> stack = new Stack<>();
        while (!stack.isEmpty() || node != null) {
            while (node != null) {
                stack.push(node);
                node = node.getLeft();
            }
            if (!stack.isEmpty()) {
                TreeNode item = stack.pop();
                System.out.println(item.getValue());
                node = item.getRight();
            }
        }
    }

    // 非递归后序遍历
    private static void posOrder(TreeNode head) {
        if (head == null) {
            return;
        }
        Stack<TreeNode> s1 = new Stack<>();
        Stack<TreeNode> s2 = new Stack<>();
        s1.push(head);
        while (!s1.isEmpty()) {
            TreeNode cur = s1.pop();
            s2.push(cur);
            if (cur.left != null) {
                s1.push(cur.left);
            }
            if (cur.right != null) {
                s1.push(cur.right);
            }
        }
        while (!s2.isEmpty()) {
            TreeNode cur = s2.pop();
            System.out.println(cur.getValue());
        }
    }


    private static void levelOrder(TreeNode node) {
        if (node == null) {
            return;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(node);
        while (!queue.isEmpty()) {
            TreeNode item = queue.poll();
            System.out.println(item.getValue());
            if (item.getLeft() != null) {
                queue.add(item.getLeft());
            }
            if (item.getRight() != null) {
                queue.add(item.getRight());
            }
        }
    }

}