Serialize and Deserialize Binary Tree

1. Use LinkedList as a global buffer

/**
 * 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.
    private void serialize(TreeNode n, StringBuilder sb){
        if(n == null){
            sb.append("null").append("#");
            return;
        } 
        sb.append(n.val).append("#");
        serialize(n.left, sb);
        serialize(n.right, sb);
    }
    public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        serialize(root, sb);
        return sb.toString();
    }

    private TreeNode deserialize(LinkedList<String> strs){
        String crt = strs.getFirst();
        strs.removeFirst();
        if (crt.equals("null")){
            return null;
        }
        TreeNode n = new TreeNode(Integer.parseInt(crt));
        n.left = deserialize(strs);
        n.right = deserialize(strs);
        return n;
    }
    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        LinkedList<String> strs = new LinkedList<>(Arrays.asList(data.split("#")));
        return deserialize(strs);
    }
}

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