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.

Friend: 449. Serialize and Deserialize BST

Solution：Preorder 序列化 + 单queue 递归反序列化

思路：

Screen Shot 2017-12-01 at 21.43.47.png

BT方法：BT/BST通用，稍废Space，但Time：O(N)
BST方法：BST独用(根据BST特性)，可省Space，但Time for Deserialize最差有可能：O(N^2)

Time Complexity: O(N) Space Complexity: O(N)

Solution Code：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {
    private static final String SEP = ",";
    private static final String NN = "#";

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if(root == null) return null;
        StringBuilder sb = new StringBuilder();
        buildString(root, sb);
        System.out.println("String: " + sb.toString());
        return sb.toString();
    }

    private void buildString(TreeNode node, StringBuilder sb) {
        if (node == null) {
            sb.append(NN).append(SEP);
        } else {
            sb.append(node.val).append(SEP);
            buildString(node.left, sb);
            buildString(node.right,sb);
        }
    }
    
    
    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data == null) return null;
        
        Queue<String> queue = new LinkedList<>();
        String[] strs = data.split(SEP);
        for (String e : strs) {
            queue.offer(e);
        }
        
        return buildTree(queue);
    }
    
    private TreeNode buildTree(Queue<String> queue) {
        if (queue.isEmpty()) return null;
        String val = queue.poll();
        if (val.equals(NN)) return null;
        else {
            TreeNode node = new TreeNode(Integer.valueOf(val));
            node.left = buildTree(queue);
            node.right = buildTree(queue);
            return node;
        }
    }
}

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