[LeetCode 297] 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.

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.

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此题有两种解法：一种是常规的用队列实现的解法； 一种是用C++的ostringstream函数的递归方法。后一种方法比较简练，值得学习：

第一种：

队列

/**
 * 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;
        if(!root) return res;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()){
            TreeNode *cur = q.front();
            q.pop();
            if(!cur) res += "#,";
            else{
                res += to_string(cur->val) + ",";
                q.push(cur->left);
                q.push(cur->right);
            }
        }
        cout << res;
        return res;
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        TreeNode *root = nullptr;
        if(data.empty()) return root;
        queue<TreeNode*> q;
        for(int i=0; i<data.size(); i++){
            int pos = data.find_first_of(',',i);
            string cur = data.substr(i, pos-i);
            if(q.empty()){
                root = new TreeNode(stoi(cur));
                q.push(root);
                i = pos;
            }else{
                TreeNode *node = q.front();
                q.pop();
                if(cur != "#"){
                    TreeNode *newnode = new TreeNode(stoi(cur));
                    node->left = newnode;
                    q.push(newnode);
                }
                int pos2 = data.find_first_of(',',pos+1);
                cur = data.substr(pos+1, pos2-pos-1);
                if(cur != "#"){
                    TreeNode *newnode = new TreeNode(stoi(cur));
                    node->right = newnode;
                    q.push(newnode);
                }
                i = pos2;
            }
        }
        return root;
    }
};

// Your Codec object will be instantiated a

第二种：

class Codec {
public:

    string serialize(TreeNode* root) {
        ostringstream out;
        serialize(root, out);
        return out.str();
    }

    TreeNode* deserialize(string data) {
        istringstream in(data);
        return deserialize(in);
    }

private:

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

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