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  • [LeetCode] 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:

    解法一:

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

    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:

    解法二:

    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;
            TreeNode *res = new TreeNode(stoi(val)), *cur = res;
            q.push(cur);
            while (!q.empty()) {
                TreeNode *t = q.front(); q.pop();
                if (!(in >> val)) break;
                if (val != "#") {
                    cur = new TreeNode(stoi(val));
                    q.push(cur);
                    t->left = cur;
                }
                if (!(in >> val)) break;
                if (val != "#") {
                    cur = new TreeNode(stoi(val));
                    q.push(cur);
                    t->right = cur;
                }
            }
            return res;
        }
    };

    参考资料:

    https://leetcode.com/problems/serialize-and-deserialize-binary-tree/

    https://leetcode.com/problems/serialize-and-deserialize-binary-tree/discuss/74253/Easy-to-understand-Java-Solution

    https://leetcode.com/problems/serialize-and-deserialize-binary-tree/discuss/74261/Easy-to-understand-java-solution

    https://leetcode.com/problems/serialize-and-deserialize-binary-tree/discuss/74264/Short-and-straight-forward-BFS-Java-code-with-a-queue

    LeetCode All in One 题目讲解汇总(持续更新中...)

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  • 原文地址:https://www.cnblogs.com/grandyang/p/4913869.html
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