LeetCode 449. Serialize and Deserialize BST 序列化和反序列化二叉搜索树 (Java)

题目：

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.

分析：

序列化一颗二叉搜索树，可以和序列化反序列化二叉树用相同的做法，这次就用层次遍历来做。

程序：

/**
 * 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 str = new StringBuilder();
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()){
            int len = queue.size();
            LinkedList<TreeNode> temp = new LinkedList<>();
            for(int i = 0; i < len; ++i){
                TreeNode node = queue.removeFirst();
                if (node != null) {
                    temp.add(node.left);
                    temp.add(node.right);
                    str.append(node.val).append(",");
                } else {
                    str.append("#,");
                }
            }
            queue = temp;
            //System.out.println(queue.toString());
        }
        return str.toString();
    }
    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data == "" || data == null)
            return null;
        String[] strs = data.split(",");
        Queue<String> l = new ArrayDeque<>(Arrays.asList(strs));
        Queue<TreeNode> nodes = new ArrayDeque<>();
        TreeNode root = new TreeNode(Integer.parseInt(l.poll()));
        nodes.offer(root);
        while(!nodes.isEmpty() && !l.isEmpty()){
            TreeNode node = nodes.poll();
            String strl = l.poll();
            if(!strl.equals("#")){
                node.left = new TreeNode(Integer.parseInt(strl));
                nodes.offer(node.left);
            }
            String strr = l.poll();
            if(!strr.equals("#")){
                node.right = new TreeNode(Integer.parseInt(strr));
                nodes.offer(node.right);
            }
        }
        return root;
    }

}

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