leetcode297. Serialize and Deserialize Binary Tree

题目要求

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.

设计一个方法将一个二叉树序列化并反序列化。序列化是指将对象转化成一个字符串，该字符串能够在网络上传输，而且到达目的地后能够经过反序列化恢复成原来的对象。

在Leetcode中，树的序列化是经过广度优先遍历实现的，就如题目中所介绍的那样。

这里咱们使用中序遍从来解决这个问题。

思路和代码

中序遍历是指递归的先遍历当前节点，而后按一样的方式递归的遍历左子树，再递归的遍历右子树。所以题目中的例子的中序遍历结果为[1,2,3,4,5]。可是，标准的中序遍历并不能使咱们将该结果反构形成一棵树，咱们丢失了父节点和子节点之间的关系。所以咱们须要适当的插入空值来保存父节点和子节点的关系。

所以，咱们将访问到空值也记录下来，结果以下[1,2, , ,3,4, , ,5, , ]。

这里咱们也能够明显的看出来，中序遍历须要保存的空值远远多于广度优先遍历。

那么咱们如何将这个字符串反序列化呢？
其实思路仍是同样的，将其根据分隔符分割后传入队列，不断从队列头读取数据，先构建当前节点，而后依次构建左子树和右子树，若是遇到空值，则返回递归。

代码以下：

private static final String NULL = "N";
    private static final String SPLITOR = ",";
    
     // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder result = new StringBuilder();
        inOrder(root, result);
        return result.toString();
    }
    
    private void inOrder(TreeNode root, StringBuilder result){
        if(root==null){
            result.append(NULL).append(SPLITOR);
        }else{
            result.append(root.val).append(SPLITOR);
            inOrder(root.left, result);
            inOrder(root.right, result);
        }
    }

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

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