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.

Solution

/**
 * 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";
        StringBuilder sb = new StringBuilder();

        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        Queue<TreeNode> next = new LinkedList<TreeNode>();
        queue.offer(root);
        sb.append(root.val);
        while(!queue.isEmpty()) {
            // every good father will have their children serialize
            while(!queue.isEmpty()) {
                TreeNode node = queue.poll();
                if(node != null) {
                    // serialize its children
                    sb.append("," + (node.left == null ? "n" : node.left.val));
                    sb.append("," + (node.right == null ? "n" : node.right.val));
                    if(node.left != null) next.offer(node.left);
                    if(node.right != null) next.offer(node.right);
                }
            }
            Queue tmp = queue; queue = next; next = tmp;
        }
        return sb.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data == null || data.equals("null")) return null;
        String[] nodes = data.split(",");

        TreeNode root = new TreeNode(Integer.valueOf(nodes[0]));
        List<TreeNode> list = new ArrayList<TreeNode>();
        list.add(root);

        int listIdx = 0;
        int i = 1;
        while(i < nodes.length) {
            if(!nodes[i].equals("n")) list.get(listIdx).left = new TreeNode(Integer.valueOf(nodes[i]));
            if(!nodes[i+1].equals("n")) list.get(listIdx).right = new TreeNode(Integer.valueOf(nodes[i+1]));
            i += 2;
            listIdx++;
            if(listIdx == list.size()) {
                List<TreeNode> nlist = new ArrayList<TreeNode>();
                for(TreeNode node: list) {
                    if(node.left != null) nlist.add(node.left);
                    if(node.right != null) nlist.add(node.right);
                }
                list = nlist;
                listIdx = 0;
            }
        }
        return root;
    }
}

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