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.

Example:

You may serialize the following tree:

    1
   / \
  2   3
     / \
    4   5

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

Solution: Preorder encoding using string stream

We use a string stream to encode and decod the tree by preorder. We also need a char to separate each node, and a char to mark the null pointer. In this solution, we choose empty space and '#'.

class Codec {
public:
    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        ostringstream out;
        serialize(root, out);
        return out;
    }
    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        istringstream in(data);
        return deserialize(in);
    }

private:
    void serialize(TreeNode *root, ostringstream &out) {
        if (root) {
            out << to_string(root->val) + ' ';
            serialize(root->left, out);
            serialize(root->right, out);
        } else {
            out << "# ";
        }
    }

    TreeNode* deserialize(istringstream &in) {
        string val;
        in >> val;
        if (val == "#") return NULL;
        TreeNode *root = new TreeNode(stoi(val));
        root->left = deserialize(in);
        root->right = deserialize(in);
        return root;
    }
};

Solution: Level order encoding and decoding

class Codec {
public:
    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        if (!root) return "";

        ostringstream s;
        queue<TreeNode *> q;
        q.push(root);
        while (!q.empty()) {
            int size = (int)q.size();
            for (int i=0; i<size; i++) {
                TreeNode *node = q.front(); q.pop();
                if (node) {
                    s << to_string(node->val) + ' ';
                    q.push(node->left);
                    q.push(node->right);
                } else {
                    s << "# ";
                }
            }
        }
        return s.str();
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        if (data.empty()) return NULL;

        istringstream in(data);
        queue<TreeNode*> q;
        string val;
        in >> val;

        TreeNode *res = new TreeNode(stoi(val)), *cur = res;
        q.push(cur);

        while (!q.empty()) {
            TreeNode *t = q.front(); q.pop();

            // Left child
            if (!(in >> val)) break;
            if (val != "#") {
                cur = new TreeNode(stoi(val));
                q.push(cur);
                t->left = cur;
            }

            // Right child
            if (!(in >> val)) break;
            if (val != "#") {
                cur = new TreeNode(stoi(val));
                q.push(cur);
                t->right = cur;
            }
        }
        return res;
    }
};