LeetCode Top 100 Liked Questions 297. Serialize and Deserialize Binary Tree (Java版; Hard)

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LeetCode Top 100 Liked Questions 297. Serialize and Deserialize Binary Tree (Java版; Hard)

题目描述

Serialization is the process of converting a data structure or object into a sequence of bits so that it can be stored in a file or memory buffer, 
or transmitted across a network connection link to be reconstructed later in the same or another computer environment.

Design an algorithm to serialize and deserialize a binary tree. There is no restriction on how your serialization/deserialization algorithm should
work. You just need to ensure that a binary tree can be serialized to a string and this string can be deserialized to the original tree structure.

Example:

You may serialize the following tree:

    1
   / \
  2   3
     / \
    4   5

as "[1,2,3,null,null,4,5]"
Clarification: The above format is the same as how LeetCode serializes a binary tree. You do not necessarily need to follow this format, so please 
be creative and come up with different approaches yourself.

Note: Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.

剑指offer上也有这个题, 我的Deserialize(String str)写法稍有不同, 具体就是LeetCode上用了String[], 剑指offer时用的是逐个char扫描,再用Integer.parseInt(); 下面是在牛客上的写法

/*
结构信息的保存, 体现在空节点的处理上: 一定要以某种形式保存空节点!
*/
public class Solution {
    String Serialize(TreeNode root) {
        if(root==null)
            return "#";
        //前序: 根左右
        return root.val+"!" + Serialize(root.left) + Serialize(root.right);
  }
    int index = 0;
    TreeNode Deserialize(String str) {
       if(str==null || index==str.length())
           return null;
        if(str.charAt(index)=='#'){
            index++;
            return null;
        }
        int i;
        String curr="";
        //这个while不用加越界判断index<str.length(); 因为序列化的过程一定在数字后面有个!
        while(str.charAt(index)!='!'){
            curr += str.charAt(index);
            index++;
        }
        //跳到!下一个位置
        index++;
        TreeNode root = new TreeNode(Integer.valueOf(curr));
        root.left = Deserialize(str);
        root.right = Deserialize(str);
        return root;
  }
}

第一次做; 注意两个核心:1)null后面必须加逗号 2)curr不能调用trim()方法

public class Codec {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if(root==null)
            //核心1: null后面必须加逗号, 为了保留结构信息; 起初我误以为递归函数非base case部分会给null加上逗号, 再一看逻辑, 非base case部分只给非null节点加逗号了! 注意逻辑啊少年!
            return "null,";
        String curr = ""+root.val+",";
        curr = curr+serialize(root.left);
        curr = curr+serialize(root.right);
        //return curr.trim();
        //核心2: curr不能调用trim()方法, 会删除掉结构信息! 不要一股脑想着怎么美化, 从而忽略了核心, 捡了芝麻丢了瓜, 大忌!
        return curr;
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        String[] str = data.split(",");
        TreeNode root = core(str);
        return root;
    }
    public int index=0;
    public TreeNode core(String[] str){
        if(index==str.length)
            return null;
        if(str[index].equals("null")){
            index++;
            return null;
        }
        TreeNode curr = new TreeNode(Integer.parseInt(str[index]));
        index++;
        curr.left = core(str);
        curr.right = core(str);
        return curr;
    }
}

// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));