297. Serialize and Deserialize Binary Tree

• Difficulty: Hard

• Topics: Tree, Design

• Similar Questions:

  • Encode and Decode Strings

  • Serialize and Deserialize BST

  • Find Duplicate Subtrees

  • Serialize and Deserialize N-ary Tree

Problem:

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.

Example: 

You may serialize the following tree:

    1
   / \
  2   3
     / \
    4   5

as "[1,2,3,null,null,4,5]"

Clarification: The above format is the same as how LeetCode 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.

Solutions:

/**
 * 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 ret;
        serializeHelper(root, ret);
        return ret;
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        int pos = 0;
        return deserializeHelper(data, pos);
    }

private:
    void serializeHelper(TreeNode* root, string& ret) {
        const static string NULL_STRING = "NULL"; 
        if (root == nullptr) {
            ret.append(NULL_STRING);
            ret.push_back(' ');
            return;
        }

        ret.append(to_string(root->val));
        ret.push_back(' ');
        serializeHelper(root->left, ret);
        serializeHelper(root->right, ret);
    }

    TreeNode* deserializeHelper(string& data, int& pos) {
        string token = getToken(data, pos);
        if (token == "NULL") {
            return nullptr;
        }

        TreeNode* root = new TreeNode(stoi(token));
        root->left = deserializeHelper(data, pos);
        root->right = deserializeHelper(data, pos);
        return root;
    }

    string getToken(string& data, int& pos) {
        string ret;
        for (;data[pos] != ' '; ++pos) {
            ret.push_back(data[pos]);
        }

        ++pos; // remove space
        return ret;
    }
};

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