297. Serialize and Deserialize Binary Tree

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.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {
    public String serialize(TreeNode root) {
        if (root == null) return "#";
        return root.val + "," + serialize(root.left) + "," + serialize(root.right);
    }

    public TreeNode deserialize(String data) {
        Queue<String> queue = new LinkedList<>(Arrays.asList(data.split(",")));
        return helper(queue);
    }
    
    private TreeNode helper(Queue<String> queue) {
        String s = queue.poll();
        if (s.equals("#")) return null;
        TreeNode root = new TreeNode(Integer.valueOf(s));
        root.left = helper(queue);
        root.right = helper(queue);
        return root;
    }
}

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

public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        serialize(root, sb);
        return sb.toString();
    }
    
    public void serialize(TreeNode root, StringBuilder sb) {
        if (root == null) {
            sb.append("#").append(",");
        } else {
            sb.append(root.val).append(",");
            serialize(root.left, sb);
            serialize(root.right, sb);
        }
    }

serialize用stringBuilder更快

注意一下啊，serialization的时候比如【1，2，3】，会生成[1,2,#,#,3,#,#],因为我们是按照preorder压进去的。

同理在deserialize的时候也是按preorder build tree的。