LintCode 1235: Serialize and Deserialize BST (二叉樹經典題)

1235. Serialize and Deserialize BST

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 search tree. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that a binary search tree can be serialized to a string and this string can be deserialized to the original tree structure.

The encoded string should be as compact as possible.

Note: Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.

Example
Example 1:

Input：[2,1,3]
Output：[2,1,3]
Explanation：
2
/
1 3
Example 2:

Input：[1,#,2]
Output：[1,#,2]
Explanation：
1

2

解法1：
serialize()用preOrder遍歷，如果遇到空節點，就輸出”# “，否則就輸出val + " "。
deserialize()檢查節點爲空則輸出NULL，否則遞歸調用deserialize()爲當前節點的左節點和右節點。
注意：
1）我們以前學過光靠preOrder或postOrder或inOrder是不能決定一個Binary Tree的，這裏爲什麼可以呢？因爲這裏serialize()把該Binary Tree變成了全二叉樹或滿二叉樹。對於全二叉樹或滿二叉樹，preOrder/postOrder/inOrder是可以決定這棵樹的。
2) stringStream 很有用，自動把string按“ ”split成tokens。

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    string serialize(TreeNode * root) {
        stringstream ss;
        serialize(root, ss);
//        cout<<"ss.str()="<<ss.str()<<endl;
        return ss.str();
    }

    TreeNode * deserialize(string &data) {
        stringstream ss(data);
        return deserialize(ss);
    }
    
private:
    void serialize(TreeNode * root, stringstream & ss) {
        if (!root) ss << "# ";
        else {
            ss << root->val << " ";
            serialize(root->left, ss);
            serialize(root->right, ss);
        }
    }
    
    TreeNode * deserialize(stringstream & ss) {
        string val = "";
        ss >> val;
        if (val == "#") return NULL;
  //      cout<<"val="<<val<<endl;
        TreeNode * node = new TreeNode(stoi(val));
        node->left = deserialize(ss);
        node->right = deserialize(ss);
        return node;
    }
};

解法2：
serialize()和deserialize()都是按層次遍歷來處理。
注意：
1） if (!(ss >> val)) 說明ss的token已經處理完了。

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    string serialize(TreeNode * root) {
        if (!root) return "";
        stringstream ss;
        queue<TreeNode *> q;
        q.push(root);
        while(!q.empty()) {
            TreeNode * node = q.front();
            q.pop();
            if (node) {
                ss << node->val << " ";
                q.push(node->left);
                q.push(node->right);
            } else {
                ss << "# ";
            }
        }
     //   cout<<"ss.str()="<<ss.str()<<endl;
        return ss.str();
    }

    TreeNode * deserialize(string &data) {
        if (data.empty()) return NULL;
        stringstream ss;
        ss << data;   //stringstream ss(data) is also OK
        queue<TreeNode *> q;
        string val = "";
        ss >> val; // the first token
        TreeNode * resNode = new TreeNode(stoi(val));
        TreeNode * curNode;
        
        q.push(resNode);
        while(!q.empty()) {
            TreeNode * node = q.front();
            q.pop();
            if (!(ss >> val)) break;  //ss reads to the end!
            if (val != "#") {
                curNode = new TreeNode(stoi(val));
                q.push(curNode);
                node->left = curNode;
            }
            if (!(ss >> val)) break;  //ss reads to the end!
            if (val != "#") {
                curNode = new TreeNode(stoi(val));
                q.push(curNode);
                node->right = curNode;
            }
        }
        return resNode;
    }
};

代碼同步在
https://github.com/luqian2017/Algorithm