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.

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.

Solution

(1) Java

/**
 * 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 sb = new StringBuilder();
        helper1(root, sb);
        sb.deleteCharAt(sb.length()-1);
        return sb.toString();
    }

    private void helper1(TreeNode root, StringBuilder sb) {
        if (root == null) {
            sb.append("null,");
            return;
        }
        sb.append(root.val+",");
        helper1(root.left, sb);
        helper1(root.right, sb);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        String[] strs = data.split(",");
        Queue<String> q = new LinkedList<>(Arrays.asList(strs));
        return helper2(q);
    }

    private TreeNode helper2(Queue<String> q) {
        if (q.size() == 0) {
            return null;
        }
        String data = q.poll();
        if (data.equals("null")) {
            return null;
        }

        TreeNode node = new TreeNode(Integer.parseInt(data));
        node.left = helper2(q);
        node.right = helper2(q);
        return node;
    }
}

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

297. Serialize and Deserialize Binary Tree