297. Serialize and Deserialize Binary Tree 二叉树的序列化和去序列化

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.

Credits:
Special thanks to @Louis1992 for adding this problem and creating all test cases.

这道题让我们对二叉树进行序列化和去序列化的操作。序列化就是将一个数据结构或物体转化为一个位序列，可以存进一个文件或者内存缓冲器中，然后通过网络连接在相同的或者另一个电脑环境中被还原，还原的过程叫做去序列化。现在让我们来序列化和去序列化一个二叉树，并给了我们例子。这题有两种解法，分别为先序遍历的递归解法和层序遍历的非递归解法。先来看先序遍历的递归解法，非常的简单易懂，我们需要接入输入和输出字符串流istringstream和ostringstream，对于序列化，我们从根节点开始，如果节点存在，则将值存入输出字符串流，然后分别对其左右子节点递归调用序列化函数即可。对于去序列化，我们先读入第一个字符，以此生成一个根节点，然后再对根节点的左右子节点递归调用去序列化函数即可，参见代码如下：

// Recursionclass Codec {public:    // Encodes a tree to a single string.    string serialize(TreeNode* root) {        ostringstream out;        serialize(root, out);        return out.str();    }    // 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 << root->val << ' ';            serialize(root->left, out);            serialize(root->right, out);        } else {            out << "# ";        }    }    TreeNode* deserialize(istringstream &in) {        string val;        in >> val;                             //逐个输入，以空格为分界符        if (val == "#") return nullptr;        TreeNode *root = new TreeNode(stoi(val));        root->left = deserialize(in);        root->right = deserialize(in);        return root;    }};

笔记：先序遍历，递归调用。

另一种方法是层序遍历的非递归解法，这种方法略微复杂一些，我们需要借助queue来做，本质是BFS算法，也不是很难理解，就是BFS算法的常规套路稍作修改即可，参见代码如下：

/** * 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) {        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);        queue<TreeNode*> q;        string val;        in >> val;                  //逐个输入，以空格为分界符        TreeNode* res = new TreeNode(stoi(val)), *cur = res;        q.push(cur);        while(!q.empty()) {            TreeNode* t = q.front();            q.pop();            if(!(in >> val)) break;   //in输入完，立刻结束            if(val != "#") {                cur = new TreeNode(stoi(val));                q.push(cur);                t->left = cur;            }            if(!(in >> val)) break;   //in输入完，立刻结束            if(val != "#") {                cur = new TreeNode(stoi(val));                q.push(cur);                t->right = cur;            }        }        return res;    }};

笔记：层次遍历，在解码时注意，在in输入完后，二叉树已经生成完毕，应立刻break结束循环。