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.

解题思路

先序遍历+递归。

实现代码

C++:

// Runtime: 140 ms
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Codec {
public:
    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        string res = "";
        serializeHelper(root, res);
        return res;
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        TreeNode *root = NULL;
        deserializeHelper(root, data);
        return root;
    }

private:
    void serializeHelper(TreeNode *root, string &res)
    {
        if (root == NULL)
        {
            res += "null ";
            return;
        }
        ostringstream oss;
        oss<<root->val;
        res += oss.str();
        res += ' ';
        serializeHelper(root->left, res);
        serializeHelper(root->right, res);
    }

    void deserializeHelper(TreeNode *&root, string& data)
    {
        if (data.length() == 0)
        {
            return;
        }

        string temp = data.substr(0, data.find(' '));
        data = data.substr(data.find(' ') + 1);

        if (temp.compare("null") == 0)
        {
            root = NULL;
        }
        else
        {
            root = new TreeNode(atoi(temp.c_str()));
            deserializeHelper(root->left, data);
            deserializeHelper(root->right, data);
        }
    }
};

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

Java:

// Runtime: 446 ms
/**
 * 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) {
        StringBuilder sb = new StringBuilder();
        return serialize(root, sb);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        StringBuilder sb = new StringBuilder(data);
        TreeNode root = null;
        return deserialize(root, sb);
    }

    private String serialize(TreeNode root, StringBuilder sb) {
        if (root == null) {
            sb.append("null ");
        }
        else {
            sb.append(root.val + " ");
            serialize(root.left, sb);
            serialize(root.right, sb);
        }

        return sb.toString();
    }

    private TreeNode deserialize(TreeNode root, StringBuilder sb) {
        if (sb.length() == 0) {
            return null;
        }

        String temp = sb.substring(0, sb.indexOf(" "));
        sb = sb.delete(0, sb.indexOf(" ") + 1);
        if (temp.equals("null")) {
            root = null;
        }
        else {
            root = new TreeNode(Integer.parseInt(temp));
            root.left = deserialize(root.left, sb);
            root.right = deserialize(root.right, sb);
        }

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