449. 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.

Solution

(1) Java

/**
 * 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 sb = new StringBuilder();
        helper1(root, sb);
        sb.deleteCharAt(sb.length()-1);
        return sb.toString();
    }

    private void helper1(TreeNode root, StringBuilder sb) {
        if (root == null) {
            return;
        }
        sb.append(root.val).append(",");
        helper1(root.left, sb);
        helper1(root.right, sb);      
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data == null || data.length() == 0) {
            return null;
        }
        Deque<Integer> deq = new LinkedList<>();
        String[] strs = data.split(",");
        for (String str : strs) {
            deq.offer(Integer.parseInt(str));
        }
        return helper2(deq);
    }

    private TreeNode helper2(Deque<Integer> deq) {
        if (deq.size() == 0) {
            return null;
        }
        TreeNode node = new TreeNode(deq.poll());
        Deque<Integer> tmpQ = new LinkedList<>();
        while (deq.size() > 0) {
            if (deq.peek() < node.val) {
                tmpQ.offer(deq.poll());
            } else {
                break;
            }
        }
        node.left = helper2(tmpQ);
        node.right = helper2(deq);
        return node;
    }
}

// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));

449. Serialize and Deserialize BST