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.

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.

Credits:
Special thanks to @Louis1992 for adding this problem and creating all test cases.

其实该题目的是设计一个序列化输出，然后反序列得到原值

你可以选择tree的各种travel， 但是要保正可以得到原来的tree

我选的是level travel

/**
 * 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 arr = new StringBuilder();
        arr.append("[");
        if(root == null) return "[]";
        ArrayList<TreeNode> queue = new ArrayList<>();
        queue.add(root);
        arr.append(root.val);
        for (int i = 0; i < queue.size(); i++) {
            TreeNode node = queue.get(i);
            if(node == null) continue;
            queue.add(node.left);
            queue.add(node.right);
        }
        
        while(queue.get(queue.size()-1) == null){   //消去leaf的null
            queue.remove(queue.size()-1);
        }
         
        for(int i = 1 ; i < queue.size(); i++){
            TreeNode node = queue.get(i);
            if(node == null) 
                arr.append(",").append("null");
            else
                arr.append(",").append(node.val);
            
        }
        arr.append("]");
        System.out.println(arr.toString());
        return arr.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data == null || data.length() == 0 || data.equals("[]")) return null;
        String[] arr = data.substring(1, data.length() - 1).split(",");
        if(arr.length == 0) return null;
        ArrayList<TreeNode> list = new ArrayList<TreeNode>();
        TreeNode root = new TreeNode(Integer.parseInt(arr[0]));
        list.add(root);
        int index = 0;
        boolean isLeftNode = true;
        for(int i = 1 ; i < arr.length ; i++){
            if(!arr[i].equals("null")){
                TreeNode node = new TreeNode(Integer.parseInt(arr[i]));
                if(isLeftNode)
                  list.get(index).left = node;
                else
                  list.get(index).right = node;
                list.add(node);
            }
            if(!isLeftNode)  index++;
            isLeftNode = !isLeftNode;
        }
        return root;
    }
}

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