LeetCode - 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 {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if(root == null){
            return "null";
        }
        return root.val + "," +serialize(root.left) + ","+serialize(root.right);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data == null || data.length() == 0){
            return null;
        }
        Queue<String> queue =new LinkedList<>();
        queue.addAll(Arrays.asList(data.split(",")));
        //String[] nodes = data.split(",");
        return deserializeHelper(queue);
    }
    public TreeNode deserializeHelper(Queue<String> nodes){
        String val = nodes.remove();
        if (val.equals("null")) return null;
        else {
            TreeNode node = new TreeNode(Integer.valueOf(val));
            node.left = deserializeHelper(nodes);
            node.right = deserializeHelper(nodes);
            return node;
        }
    }
    
}

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

　　无法直接传入int因为它是primitive type， changes made to it will not be visible outside the function scope. 

/**
 * 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 "null";
        }
        return root.val + "," +serialize(root.left) + ","+serialize(root.right);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data == null || data.length() == 0){
            return null;
        }
        //Queue<String> queue =new LinkedList<>();
        //queue.addAll(Arrays.asList(data.split(",")));
        String[] nodes = data.split(",");
        return deserializeHelper(nodes, new int[]{0});
    }
    /*
    public TreeNode deserializeHelper(Queue<String> nodes){
        String val = nodes.remove();
        if (val.equals("null")) return null;
        else {
            TreeNode node = new TreeNode(Integer.valueOf(val));
            node.left = deserializeHelper(nodes);
            node.right = deserializeHelper(nodes);
            return node;
        }
    }
    */
    
    public TreeNode deserializeHelper(String[] nodes, int[] index){
        String val = nodes[index[0]];
        index[0] = index[0]+1;
        if(val.equals("null") ){
            return null;
        }
        TreeNode node = new TreeNode(Integer.valueOf(val));
        node.left = deserializeHelper(nodes,index);
        node.right = deserializeHelper(nodes,index);
        return node;
    }
    
}

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