【二叉树序列化&反序列化】297. Serialize and Deserialize Binary Tree

问题：

实现下面两个转换：

TreeNode* -> string

string -> TreeNode*

Example 1:
Input: root = [1,2,3,null,null,4,5]
Output: [1,2,3,null,null,4,5]

Example 2:
Input: root = []
Output: []

Example 3:
Input: root = [1]
Output: [1]

Example 4:
Input: root = [1,2]
Output: [1,2]
 
Constraints:
The number of nodes in the tree is in the range [0, 104].
-1000 <= Node.val <= 1000

　　

解法：Binary Tree（二叉树）

方法一：preorder (先序遍历)（递归）

⚠️ 注意：构建二叉树，一般需要 先序+中序 or 后序+中序 三种遍历方法的其中两种，才能构建。

特别的，如果给出了叶子节点的null状况，则可由三种遍历方法的任意一种，构建。

♻️ 优化：

这里，

c++没有字符串分割函数split，我们利用输入输出流进行替代。

特点：

• TreeNode* -> string
  • ostringstream out << val
    • ostringstream -> string 方法： out.str()
• string -> TreeNode*
  • istringstream in >> val
    • 默认使用空格，进行分割读取。
    • string -> istringstream 构造方法：in(string data)

代码参考：

• TreeNode* -> string
  • ostringstream out << val
    • ostringstream -> string 方法： out.str()

class Codec {
public://Preorder
    string NUL = "#";
    string SEP = " ";

    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        ostringstream code;
        serialize_code(root, code);
        return code.str();
    }
    void serialize_code(TreeNode* root, ostringstream& code) {
        if(!root) {
            code << NUL << SEP;
            return;
        }
        //root
        code << root->val << SEP;
        //child
        serialize_code(root->left, code);
        serialize_code(root->right, code);
    }
};

• string -> TreeNode*
  • istringstream in >> val
    • 默认使用空格，进行分割读取。
    • string -> istringstream 构造方法：in(string data)

class Codec {
public://Preorder
    string NUL = "#";
    string SEP = " ";

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        istringstream code(data);
        return deserialize_code(code);
    }
    TreeNode* deserialize_code(istringstream& code) {
        string val;
        code >> val;
        if(val == NUL) return nullptr;

        TreeNode* root = new TreeNode(stoi(val));
        root->left = deserialize_code(code);
        root->right = deserialize_code(code);
        return root;
    }
};

方法二：BFS (层序遍历，广度优先搜索)（循环）

主要思想：

使用queue对TreeNode进行保存，

处理node，弹出node，pop(node)

得到node->left, node->right，并入队push(node->left), push(node->right)

代码参考：

• TreeNode* -> string
  • ostringstream out << val

class Codec {
public://BFS
    string NUL = "#";
    string SEP = " ";

    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        if(!root) return NUL;
        queue<TreeNode*> q; //queue
        ostringstream out;
        q.push(root);
        while(!q.empty()) {
            TreeNode* node = q.front();
            q.pop();
            if(!node) out << NUL << SEP;
            else {
                out << node->val << SEP;
                q.push(node->left);
                q.push(node->right);
            }
        }
        return out.str();
    }
};

• string -> TreeNode*
  • istringstream in >> val

class Codec {
public://BFS
    string NUL = "#";
    string SEP = " ";

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        istringstream in(data);
        queue<TreeNode*> q;  //queue
        TreeNode* root;
        string val;
        in >> val;
        if(val==NUL) return nullptr;
        q.push(new TreeNode(stoi(val)));
        root = q.front();
        
        while(!q.empty()) {
            TreeNode* node = q.front();
            q.pop();
            if(!node) continue;
            in >> val;
            if(val==NUL) node->left=nullptr;
            else node->left=new TreeNode(stoi(val));
            in >> val;
            if(val==NUL) node->right=nullptr;
            else node->right=new TreeNode(stoi(val));
            q.push(node->left);
            q.push(node->right);
        }
        
        return root;
    }
};

参考：stringstream 实现字符串分隔

stringstream默认分割空格、tab、回车换行 

#include <bits/stdc++.h>
using namespace std;
int main()
{
    string s1, s2;
    s1 = "Those who cannot remember the past are condemned to repeat it";
    stringstream ss(s1);
    while(ss >> s2)
        cout << s2 << endl;
}
/*
输出：
Those
who
cannot
remember
the
past
are
condemned
to
repeat
it
*/

利用指定字符分割字符串

#include <bits/stdc++.h>
using namespace std;
int main()
{
    string s1, s2;
    s1 = "Those*who*cannot*remember*the*past*are*condemned*to*repeat*it";
    stringstream ss(s1);
    while(getline(ss, s2, '*'))
        cout << s2 << endl;
}
/*
输出：
Those
who
cannot
remember
the
past
are
condemned
to
repeat
it
*/