Leetcode: Serialize and Deserialize BST

Posted 2020-08-24 neverlandly

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.

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Codec {
11 
12     // Encodes a tree to a single string.
13     public String serialize(TreeNode root) {
14         if (root == null) return "";
15         StringBuilder res = new StringBuilder();
16         Stack<TreeNode> stack = new Stack<TreeNode>();
17         TreeNode node = root;
18         while (!stack.isEmpty() || node!=null) {
19             if (node != null) {
20                 res.append(node.val).append(" ");
21                 stack.push(node);
22                 node = node.left;
23             }
24             else {
25                 node = stack.pop().right;
26             }
27         }
28         return res.toString().trim();
29     }
30 
31     // Decodes your encoded data to tree.
32     public TreeNode deserialize(String data) {
33         if (data==null || data.length()==0) return null;
34         String[] nodes = data.split(" ");
35         Queue<Integer> queue = new LinkedList<>();
36         for (String node : nodes) {
37             queue.offer(Integer.valueOf(node));
38         }
39         return buildTree(queue);
40     }
41     
42     public TreeNode buildTree(Queue<Integer> queue) {
43         if (queue.isEmpty()) return null;
44         TreeNode root = new TreeNode(queue.poll());
45         Queue<Integer> leftNodeVals = new LinkedList<>();
46         while (!queue.isEmpty() && queue.peek()<=root.val) {
47             leftNodeVals.offer(queue.poll());
48         }
49         root.left = buildTree(leftNodeVals);
50         root.right = buildTree(queue);
51         return root;
52     }
53 }
54 
55 // Your Codec object will be instantiated and called as such:
56 // Codec codec = new Codec();
57 // codec.deserialize(codec.serialize(root));