算法从下到上打印二叉树

Posted 2022-09-21 Kant101

1. 概述

OJ: https://www.nowcoder.com/questionTerminal/ed982e032ff04d6a857b4cb4e6369d04?orderByHotValue=1&mutiTagIds=583&page=3&onlyReference=false

从下到上打印二叉树属于树的遍历的改编。

2. 分析

这个问题是树的层次遍历的变种，我们可以首先通过从上到下的层次遍历得到遍历的结果，然后翻转得到从下到上的遍历结果，即为最终的结果。

3. 代码及测试

package cn.pku.edu.algorithm.leetcode.plus;

import cn.pku.edu.algorithm.leetcode.common.TreeNode;

import java.util.*;

/**
 * @author allen
 * @date 2022/9/19
 */
public class LevelOrderBottom 

    public static void main(String[] args) 
        TreeNode root = new TreeNode(1);
        TreeNode treeNode2 = new TreeNode(2);
        TreeNode treeNode3 = new TreeNode(3);
        TreeNode treeNode4 = new TreeNode(4);
        TreeNode treeNode5 = new TreeNode(5);
        TreeNode treeNode6 = new TreeNode(6);

        root.left = treeNode2;
        root.right = treeNode3;

        treeNode2.left = treeNode4;

        treeNode3.left = treeNode5;
        treeNode3.right = treeNode6;

        LevelOrderBottom levelOrderBottom = new LevelOrderBottom();
        int[][] res = levelOrderBottom.levelOrderBottom(root);
        for (int[] level: res) 
            for (int num: level) 
                System.out.print(num + " ");
            
            System.out.println();
        
    

    public int[][] levelOrderBottom(TreeNode root) 
        List<int[]> out = new ArrayList<>();
        int[] level = new int[1];
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int cur = 1, next = 0;
        int index = 0; // index of current level
        while (!queue.isEmpty()) 
            TreeNode node = queue.poll();
            level[index++] = node.val;
            cur--;
            if (node.left != null) 
                queue.add(node.left);
                next++;
            
            if (node.right != null) 
                queue.add(node.right);
                next++;
            
            if (cur == 0) 
                out.add(level);
                cur = next;
                next = 0;
                level = new int[cur];
                index = 0;
            
        
        int[][] res = new int[out.size()][];
        index = 0; // index of level
        for (int i = out.size() - 1; i >= 0; i--) 
            res[index++] = out.get(i);
        
        return res;