codec.java

public class Codec {
	public static class TreeNode {
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode(int x) { val = x; }
  }

    // Encodes a tree to a single string.
	public String serialize(TreeNode root) {
		return serialize(root, "");
	}

	public String serialize(TreeNode root, String res) {
		if (root == null) {
			return "null";
		}
		res += String.format("%s,%s,%s", Integer.toString(root.val), serialize(root.left, res), serialize(root.right, res));
		return res;
	}

	// Decodes your encoded data to tree.
	public TreeNode deserialize(String data) {
		List<Integer> vals = Arrays.asList(data.split(",")).stream()
			.map(s -> {
				if (s.equals("null")) {
					return null;
				} else {
					return Integer.parseInt(s);
				}
			})
			.collect(Collectors.toList());
		return deserialize(vals);
	}

	public TreeNode deserialize(List<Integer> data) {
		if (data.get(0) == null) {
			data.remove(0);
			return null;
		}
		TreeNode node = new TreeNode(data.remove(0));
		node.left = deserialize(data);
		node.right = deserialize(data);
		return node;
	}
}