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 tree. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that a binary tree can be serialized to a string and this string can be deserialized to the original tree structure

Example 1:

Input: root = [1,2,3,null,null,4,5]
Output: [1,2,3,null,null,4,5]

My Solutions:

用preorder的方法。

public class Codec {
    
    private final String spliter = ",";
    private final String nul = "X";

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        preOrderToBuildString(root, sb);
        return sb.toString();
    }
    
    private void preOrderToBuildString(TreeNode node, StringBuilder sb) {
        if (node == null) sb.append(nul).append(spliter);
        else {
            sb.append(node.val).append(spliter);
            preOrderToBuildString(node.left, sb);
            preOrderToBuildString(node.right, sb);
        }
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        Queue<String> nodes = new LinkedList<>();
        nodes.addAll(Arrays.asList(data.split(spliter)));
        return buildTree(nodes);
    }
    
    private TreeNode buildTree(Queue<String> nodes) {
        String val = nodes.poll();
        if (val.equals(nul)) return null;
        else {
            TreeNode node = new TreeNode(Integer.valueOf(val));
            node.left = buildTree(nodes);
            node.right = buildTree(nodes);
            return node;
        }
    }
}