297. 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.

好像想复杂了，最初的想法是前序遍历加上中序遍历或者后序遍历加上中序遍历，这样才可以确定一颗二叉树，但是这样的前提是二叉树中必须没有重复的节点

参见

https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/description/

https://leetcode.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal/description/

然后就想到用层次遍历 比如上图的二叉树 保存为1,2,3,null,null,4,5 但是最后testcase超时了 因为是下面这种情况

      1

     / 

    2  

   /  

  3

 /

4

.....

我会保存为 1,2,null,3,null,null,null,4....

实际上，仅用前序遍历也是可以的

The idea is simple: print the tree in pre-order traversal and use "X" to denote null node and split node with ",". We can use a StringBuilder for building the string on the fly. For deserializing, we use a Queue to store the pre-order traversal and since we have "X" as null node, we know exactly how to where to end building subtress.

private static final String spliter = ",";    private static final String NN = "X";    // 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(NN).append(spliter);        } else {            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) {        Deque<String> nodes = new LinkedList<>();        nodes.addAll(Arrays.asList(data.split(spliter)));        return buildTree(nodes);    }        private TreeNode buildTree(Deque<String> nodes) {        String val = nodes.remove();        if (val.equals(NN)) return null;        else {            TreeNode node = new TreeNode(Integer.valueOf(val));            node.left = buildTree(nodes);            node.right = buildTree(nodes);            return node;        }    }