Leetcode: Serialize and Deserialize BST

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 search tree. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that a binary search tree can be serialized to a string and this string can be deserialized to the original tree structure.
 
The encoded string should be as compact as possible.
 
Note: Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.

So the first question is: what is the difference between this and #297?

This here is BST, however, in #297, it's BT. "The encoded string should be as compact as possible" here. The special property of binary search trees compared to general binary trees allows a more compact encoding. So while the solutions to problem #297 still work here as well, they're not as good as they should be.

For general BT, to reconstruct the tree, we need the information of both in-order and pre-order, or in-order and post-order. We've practised that already.

However, as a BST, just the information of pre-order or post-order is sufficient to rebuild the tree.

The encoded string is also most compact, since we do not need to keep tract of information of 'Null nodes'.

Example:   4

　　　　 2       6

1   3   5    7

The pre-order encoding is: "4213657". It is easy to tell "4" is root, "213" is left tree, "657" is right tree. We can use a Queue to implement this, very convenient.

 /**
  * 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 "";
         StringBuilder res = new StringBuilder();
         Stack<TreeNode> stack = new Stack<TreeNode>();
         TreeNode node = root;
         while (!stack.isEmpty() || node!=null) {
             if (node != null) {
                 res.append(node.val).append(" ");
                 stack.push(node);
                 node = node.left;
             }
             else {
                 node = stack.pop().right;
             }
         }
         return res.toString().trim();
     }
 
     // Decodes your encoded data to tree.
     public TreeNode deserialize(String data) {
         if (data==null || data.length()==0) return null;
         String[] nodes = data.split(" ");
         Queue<Integer> queue = new LinkedList<>();
         for (String node : nodes) {
             queue.offer(Integer.valueOf(node));
         }
         return buildTree(queue);
     }
 
     public TreeNode buildTree(Queue<Integer> queue) {
         if (queue.isEmpty()) return null;
         TreeNode root = new TreeNode(queue.poll());
         Queue<Integer> leftNodeVals = new LinkedList<>();
         while (!queue.isEmpty() && queue.peek()<=root.val) {
             leftNodeVals.offer(queue.poll());
         }
         root.left = buildTree(leftNodeVals);
         root.right = buildTree(queue);
         return root;
     }
 }
 
 // Your Codec object will be instantiated and called as such:
 // Codec codec = new Codec();
 // codec.deserialize(codec.serialize(root));