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.

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.

序列化一个二叉树，然后再根据二叉树的序列得到二叉树。我们可以用层序遍历(队列)得到一棵树的序列化，空的节点用n表示，然后在根据序列化生成二叉树，同样借助队列。代码如下：

/**
 * 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) {
        if(root == null) return "";
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        StringBuilder sb = new StringBuilder();
        queue.offer(root);
        while(!queue.isEmpty()) {
            TreeNode node = queue.poll();
            if(node == null) {
                sb.append("n ");
            } else {
                sb.append(node.val + " ");
                queue.offer(node.left);
                queue.offer(node.right);
            }
        }
        return sb.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data == "") return null;
        String[] string = data.split("\\s");
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        TreeNode root = new TreeNode(Integer.parseInt(string[0]));
        queue.offer(root);
        for(int i = 1; i < string.length; i++) {
            TreeNode parent = queue.poll();
            if(!string[i].equals("n")) {
                 parent.left = new TreeNode(Integer.parseInt(string[i]));
                 queue.offer(parent.left);
            }
            if(++i < string.length && !string[i].equals("n")) {
                parent.right = new TreeNode(Integer.parseInt(string[i]));
                queue.offer(parent.right);
            }
        }
        return root;
    }
}

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

我们还可以通过DFS得到一个序列，然后在生成二叉树。代码如下：

/**
 * 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) {
        if(root == null) return "";
        Stack<TreeNode> stack = new Stack<TreeNode>();
        StringBuilder sb = new StringBuilder();
        stack.push(root);
        while(!stack.isEmpty()) {
            TreeNode parent = stack.pop();
            if(parent == null) {
                sb.append("n ");
            } else {
                sb.append(parent.val + " ");
                stack.push(parent.right);
                stack.push(parent.left);
            }
        }
        System.out.println(sb.toString());
        return sb.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data == "") return null;
        String[] string = data.split("\\s");
        int[] i = new int[1];
        return getNode(string, i);
    }
    //  1 2 n n 3 n n
    private TreeNode getNode(String[] string, int[] i) {
        if(i[0] >= string.length || string[i[0]].equals("n")) return null;
        TreeNode root = new TreeNode(Integer.parseInt(string[i[0]]));
        i[0]++;
        root.left = getNode(string, i);
        i[0]++;
        root.right = getNode(string, i);
        return root;
    }
}

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