问题：

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.与297题的区别

Note: Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.

解决：

①  与相似，但是一般的树变成了BST，而且要求是as compact as possible。还是可以用preorder，还是需要分隔符，但是null就不需要保存了。deserialize部分要变得复杂，left的值总是小于root的值，right的值总是大于root的值，根据这个每次recursion的时候把左边的值都放到另一个queue里面，剩下的就是右边的值。

public class Codec { //17ms

    // Encodes a tree to a single string.     public String serialize(TreeNode root) {         if (root == null) return "";         StringBuilder encodedStr = new StringBuilder();         encode(root,encodedStr);         return encodedStr.substring(1).toString();     }     public void encode(TreeNode root,StringBuilder sb){         if (root == null) return;         sb.append(",").append(root.val);         encode(root.left,sb);         encode(root.right,sb);     }     // Decodes your encoded data to tree.     public TreeNode deserialize(String data) {         if (data.length() == 0) return null;         Queue<Integer> queue = new LinkedList<>();         for (String s : data.split(",")){             queue.offer(Integer.valueOf(s));         }         return decode(queue);     }     public TreeNode decode(Queue<Integer> queue){         if (queue.isEmpty()) return null;         int cur = queue.poll();         TreeNode root = new TreeNode(cur);         Queue<Integer> left = new LinkedList<>();         while(! queue.isEmpty() && queue.peek() < cur){             left.offer(queue.poll());         }         root.left = decode(left);         root.right = decode(queue);         return root;     } } // Your Codec object will be instantiated and called as such: // Codec codec = new Codec(); // codec.deserialize(codec.serialize(root));