Leetcode/各大家 -- 297. Serialize and Deserialize Binary Tree (Design)

297. Serialize and Deserialize Binary Tree (Design)
Hard
https://leetcode.com/problems/serialize-and-deserialize-binary-tree/

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.

For example, you may serialize the following tree

    1
   / \
  2   3
     / \
    4   5

as "[1,2,3,null,null,4,5]", just the same as how LeetCode OJ serializes a binary tree. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.

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

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Serialize: Binary Tree 转成 String
Deserialize: String 转成 Binary Tree

思路： use 2 string marker: "X" for null - empty node, "," for spliter - spliting nodes
          To Serialize, do pre-order traversal. Append node.val(sb可直接append数字) + spliter to StringBuilder recursively.
          To Deserialize, create a queue to add nodes from string(need to 1st split 2nd convert to arraylist) . Then recursively build the tree by pointing left & right children.

Complexity: O(N) time travel all nodes

public class Codec {
    private final String nullN = "X";
    private final String spliter = ",";
    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        buildString(root,sb);
        return sb.toString();
    }
    private void buildString(TreeNode node, StringBuilder sb){
        if(node == null){
            sb.append(nullN).append(spliter);
            return;
        }
        sb.append(node.val).append(spliter);
        buildString(node.left,sb);
        buildString(node.right,sb);
    }
    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        Queue<String> nodes = new LinkedList<String>();
        nodes.addAll(Arrays.asList(data.split(spliter)));
        return buildTree(nodes);
    }
    private TreeNode buildTree(Queue<String> nodes){
        String val = nodes.remove();//return null
        if(val.equals(nullN))
            return null;
        TreeNode node = new TreeNode (Integer.valueOf(val));//Integer.parseInt()
        node.left = buildTree(nodes);
        node.right = buildTree(nodes);
        return node;
    }
}

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