src/main/java/com/fishercoder/solutions/_297.java

package com.fishercoder.solutions;

import com.fishercoder.common.classes.TreeNode;

import java.util.LinkedList;
import java.util.Queue;

/**
 * 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.

 For example, you may serialize the following tree

     1
    / \
   2   3
  / \
 4   5

 as "[1,2,3,null,null,4,5]", just the same as how LeetCode OJ 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.
 */
public class _297 {

    /**The idea is very straightforward:
     use "#" as the terminator, do BFS, level order traversal to store all nodes values into a StringBuilder.

     When deserializing, also, use a queue: pop the root into the queue first, then use a for loop to construct each node,
     then eventually just return the root.
     */

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if(root == null) return "";

        StringBuilder sb = new StringBuilder();
        Queue<TreeNode> queue = new LinkedList();
        queue.offer(root);
        while(!queue.isEmpty()){
            int size = queue.size();
            for(int i = 0; i < size; i++){
                TreeNode curr = queue.poll();
                if(curr == null){
                    sb.append("# ");
                    continue;
                }
                sb.append(curr.val);
                sb.append(" ");
                queue.offer(curr.left);
                queue.offer(curr.right);
            }
        }
        return sb.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data == null || data.isEmpty()) return null;

        String[] nodes = data.split(" ");
        TreeNode root = new TreeNode(Integer.valueOf(nodes[0]));
        Queue<TreeNode> queue = new LinkedList();
        queue.offer(root);
        for(int i = 1; i < nodes.length; i++){
            TreeNode curr = queue.poll();
            if(!nodes[i].equals("#")){
                curr.left = new TreeNode(Integer.valueOf(nodes[i]));
                queue.offer(curr.left);
            }
            if(!nodes[++i].equals("#")){
                curr.right = new TreeNode(Integer.valueOf(nodes[i]));
                queue.offer(curr.right);
            }
        }
        return root;
    }
}