二叉树的层序遍历--Java实现

package org.simpleframework.leetcode;

import java.util.*;

/**
 * @author mazhen
 * @className LevelTraversal
 * @Description 层序遍历: https://blog.csdn.net/weixin_43548025/article/details/106460961
 * @date 2021/5/13 11:13
 */
public class LevelTraversal {
    public static void main(String[] args) {

        ListNode node4 = new ListNode(4,null,null);
        ListNode node5 = new ListNode(5,null,null);
        ListNode node3 = new ListNode(3,node4,node5);
        ListNode node2 = new ListNode(2,null,null);
        ListNode node1 = new ListNode(1,node2,node3);
        //printNode(node1);
        printBootNode(node1);

    }

    //给定一个二叉树，返回该二叉树由顶层到底层的层序遍历，（从左向右，从根节点到叶子节点，一层一层的遍历）
    public static void printNode(ListNode root) {
        if (root == null) {
            return;
        }
        Queue<ListNode> queue = new LinkedList<>();
        queue.add(root);
        while (queue.size()>0) {
            ListNode node = queue.poll();
            if (node == null) {
               return;
            }
            System.out.println(node.value);
            if (node.left != null) {
                queue.add(node.left);
            }
            if (node.right != null) {
                queue.add(node.right);
            }

        }
    }

    //给定一个二叉树，返回该二叉树由底层到顶层的层序遍历，（从左向右，从叶子节点到根节点，一层一层的遍历）
    public static void printBootNode(ListNode root) {
        if (root == null) {
            return;
        }
        Stack<Integer> stack = new Stack<>();
        Queue<ListNode> queue = new LinkedList<>();
        queue.add(root);
        while (queue.size()>0) {
            ListNode node = queue.poll();
            if (node == null) {
                break;
            }
            stack.push(node.value);
            if (node.right != null) {
                queue.add(node.right);
            }
            if (node.left != null) {
                queue.add(node.left);
            }
        }
        while (!stack.empty()) {
            System.out.println(stack.pop());
        }
    }

    public static class ListNode {
        private final int value;
        private final ListNode left;
        private final ListNode right;

        public ListNode(int value,ListNode left,ListNode right) {
            this.value = value;
            this.left = left;
            this.right = right;
        }
    }

}