Binary Tree Serialization.java

class Solution {
    /**
     * This method will be invoked first, you should design your own algorithm 
     * to serialize a binary tree which denote by a root node to a string which
     * can be easily deserialized by your own "deserialize" method later.
     */
    public String serialize(TreeNode root) {
        // write your code here
        if(root == null) return "";
        StringBuilder sb = new StringBuilder();
        serialize(root, sb);
        
        return sb.toString();
    }
    public void serialize(TreeNode root, StringBuilder sb){
        if(root == null){
            sb.append("# ");
        }else{
            sb.append(root.val + " ");
            serialize(root.left, sb);
            serialize(root.right, sb);
        }
        
    }
    
    /**
     * This method will be invoked second, the argument data is what exactly
     * you serialized at method "serialize", that means the data is not given by
     * system, it's given by your own serialize method. So the format of data is
     * designed by yourself, and deserialize it here as you serialize it in 
     * "serialize" method.
     */
    public TreeNode deserialize(String data) {
        // write your code here
        if(data == null || data.length() == 0) return null;
        StringTokenizer st = new StringTokenizer(data, " ");
        return deserialize(st);
    }
    
    private TreeNode deserialize(StringTokenizer st){
        if (!st.hasMoreTokens())
            return null;
        String val = st.nextToken();
        if (val.equals("#"))
            return null;
        TreeNode root = new TreeNode(Integer.parseInt(val));
        root.left = deserialize(st);
        root.right = deserialize(st);
        return root;
    }
}