Serialize and Deserialize Binary Tree -- Leetcode

Question:

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.


Answer:
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {
    private StringBuilder str = new StringBuilder();
   
    // Encodes a tree to a single string.
    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){
            str.append("# ");
        }
        str.append(root.val + " ");
        serialize(root.left, sb);
        serialize(root.right, sb);
    }
   
   
    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data==null || data.length()==0){
            return null;
        }
        StringTokenizer st = new StringTokenizer(data, " ");
        return deserialize(st);
    }
   
    public TreeNode deserialize(StringTokenizer st){
        if(!st.hasMoreTokens()){
            return null;
        }

        String str_val = st.nextToken();
       
        if(str_val.equals("#")){
            return null;
        }
        TreeNode root = new TreeNode(Integer.parseInt(str_val));
        root.left = deserialize(st);
        root.right = deserialize(st);
        return root;
    }
}

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