【推荐】leetcode：二叉树的序列化和反序列化(先序(DFS)和按层遍历(BFS))

leetcode 297. Serialize and Deserialize Binary Tree
找工作面试的过程中碰见了一道出场率很高的面试题，在leetcode上有这道题，属于hard难度，leetcode297-实现二叉树的序列化和反序列化。用了两种方法，一种按层次(BFS)进行序列化和反序列化；另一种是按先序(DFS)进行序列化和反序列化

题目主要考察对树的遍历，以及如何根据一个输入序列构建一颗二叉树（如何构建二叉树），leetcode均测试通过。

方法一：按层次（BFS）序列化和反序列化。

package sgy.leetcodeTest;

import java.util.LinkedList;
import java.util.Queue;
import java.util.Scanner;
//节点的定义
class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }   
}

public class Codec {
    public static void main(String[] args) {
        Codec codec = new Codec();
        TreeNode root1 = codec.create();
        TreeNode root2 = codec.deserialize(codec.serialize(root1));
        codec.serialize(root2); //测试是否重建二叉树成功
    }

    //创建二叉树用于测试
    //先序建立："#" 表示 null
    // 5 2 # # 3 2 3 # # 1 # # 4 # #
    public TreeNode create() {
        Scanner sc = new Scanner(System.in);
        TreeNode root = null;
        int val = 0;
        String str = sc.next();
        if (str.equals("#"))
            root = null;
        else {
            val = Integer.parseInt(str);
            root = new TreeNode(val);
            root.left = create();
            root.right = create();
        }
        return root;
    }

    //按层次BFS的序列化
    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder res = new StringBuilder(); // 保存最终的结果
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        if (root == null)
            return res.toString();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode cur = queue.poll();
            if (cur != null) {
                res.append(cur.val + " ");
                queue.offer(cur.left); //左节点入队列
                queue.offer(cur.right); //右节点入队列
            } else {
                res.append("# ");
            }
        }
        System.out.println(res.toString().trim());
        return res.toString().trim();
    }
    //反序列化
    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data.length() == 0)
            return null;
        String[] str = data.split(" ");
        TreeNode root = createTree(str);
        return root;
    }
    //按层BFS的反序列化核心代码
    //根据层遍历的反序列化是重做层遍历，遇到"#"就生成null节点，同时不把null节点放到队列里即可
    public TreeNode createTree(String[] str) {
        TreeNode root = null;
        int index = 0;
        root = createTreeNode(str[index++]);
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        TreeNode node = null;
        while (!queue.isEmpty()) {
            node = queue.poll();
            node.left = createTreeNode(str[index++]);
            node.right = createTreeNode(str[index++]);
            if (node.left != null)
                queue.offer(node.left);
            if (node.right != null)
                queue.offer(node.right);
        }
        return root;
    }

    public TreeNode createTreeNode(String str) {
        if (str.equals("#"))
            return null;
        return new TreeNode(Integer.parseInt(str));
    }    
}

层次序列化测试截图，leetcode测试
这里写图片描述
方法二：通过先序遍历二叉树，实现二叉树的序列化和反序列化。

//按先序DFS来序列化和反序列化
public String serialize(TreeNode root) {
            StringBuilder res = new StringBuilder();
            preOrder(root, res);
            System.out.println(res.toString().trim());
            return res.toString().trim();
        }

    public TreeNode deserialize(String data) {
        if (data == null || data.length() == 0)
            return null;
        String[] str = data.split(" ");
        int[] index = new int[]{0};
        TreeNode root = reCreateTree(str, index);
        return root;
    }

    public TreeNode reCreateTree(String[] str, int[] index) {
        TreeNode root = null;
        if (index[0] < str.length) {
            int val = 0;
            if (str[index[0]].equals("#")) {
                return null;
            } else {                
                val = Integer.parseInt(str[index[0]]);
                root = new TreeNode(val);
                index[0]++;
                root.left = reCreateTree(str, index);
                index[0]++;
                root.right = reCreateTree(str, index);
            }
        }
        return root;
    }

    public void preOrder(TreeNode root, StringBuilder res) {
        if (root == null) {
            res.append("# ");
            return;
        }
        int val = root.val;
        res.append(val + " ");
        preOrder(root.left, res);
        preOrder(root.right, res);
    }

先序序列化及反序列化运行测试截图
这里写图片描述