297. Serialize and Deserialize Binary Tree (Hard)

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.

Solution 1: DFS pre-order recursive

version 1: using stringstream 29ms

class Codec {
public:
    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        if (!root) return "#"; 
        // pre-order traverse
        return to_string(root->val)+" "+serialize(root->left)+" "+serialize(root->right);
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        istringstream in(data);
        return deserialize(in);
    }
private:
    TreeNode* deserialize(istringstream &in) {
        string val;
        in >> val;
        if (val == "#") return NULL;
        TreeNode* node = new TreeNode(stoi(val));
        node->left = deserialize(in);
        node->right = deserialize(in);
        return node;
    }
};

version 2: using string 85ms

/**
 * 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) {
        if (!root) return "#";
        return to_string(root->val)+" "+serialize(root->left)+" "+serialize(root->right);
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        return buildtree(data);
    }

    TreeNode* buildtree(string& s) {
        if (s[0] == '#') {
            if (s.size() > 1) s = s.substr(2);
            return NULL;
        } 
        TreeNode* node = new TreeNode(helper(s));
        node->left = buildtree(s);
        node->right = buildtree(s);
        return node;
    }

    int helper(string& s) {
        int pos = s.find(' ');
        int val = stoi(s.substr(0,pos));
        s = s.substr(pos+1);
        return val;
    }
};

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

Solution 2: BFS iterative

class Codec {
public:
    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        ostringstream out;
        queue<TreeNode*> q;
        if (root) q.push(root);
        while (!q.empty()) {
            TreeNode* t = q.front(); q.pop();
            if (t) {
                out << t->val << " ";
                q.push(t->left);
                q.push(t->right);
            } else {
                out << "# ";
            }
        }
        return out.str();
    }

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

        istringstream in(data);
        string val; in >> val;
        TreeNode* res = new TreeNode(stoi(val));
        queue<TreeNode*> q; q.push(res);
        TreeNode* cur = res;
        while (!q.empty() && in >> val) {
            TreeNode* t = q.front(); q.pop();
            if (val != "#") {
                cur = new TreeNode(stoi(val));
                q.push(cur); t->left = cur;
            }

            in >> val;
            if (val != "#") {
                cur = new TreeNode(stoi(val));
                q.push(cur); t->right = cur;
            }

        }
        return res;
    }
};

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