[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));