[leetcode]297. Serialize and Deserialize Binary Tree 序列化与反序列化二叉树

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.

思路：

1. To serialize,  maintain a StringBuilder, using dfs to preorder traversal the tree

(1) if node is null, StringBuilder append ("#") and a splitter(",")

(2)otherwise, StringBuilder append (root.val) and a splitter(",")

2. To deserialize,  mainatain a queue containing each node informaton, using corresponding preorder traversal to build a tree 

代码：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec { 
    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        buildString(root, sb);
        return sb.toString();
    }
    
    private void buildString(TreeNode root, StringBuilder sb){
        if(root == null){
            sb.append("#").append(",");
        }else{
            // I use pre-order traversal: root, left, right 
            sb.append(root.val).append(",");
            buildString(root.left, sb);
            buildString(root.right, sb);
        }
    } 

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data == null) return null;
        String[]strArr = data.split(",");
        Queue<String> queue = new LinkedList<>();
        // put each item of strArr into queue
        Collections.addAll(queue, strArr);
        return buildTree(queue);
    }
    
    private TreeNode buildTree(Queue<String> queue){
        if(queue.isEmpty()) return null;
        String s = queue.poll();
        if(s.equals("#")) return null;
        // to match serialize pre-order traversal
        TreeNode root = new TreeNode(Integer.parseInt(s));
        root.left = buildTree(queue);
        root.right = buildTree(queue);
        return root;
    }    
}

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