作者：whisper

链接：https://www.proprogrammar.com/article/914

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剑指 Offer 37. 序列化二叉树

请实现两个函数，分别用来序列化和反序列化二叉树。

示例: 

你可以将以下二叉树：

    1
   / \
  2   3
     / \
    4   5

序列化为 "[1,2,3,null,null,4,5]"

难度：困难；标签：树，设计；编程语言：C++

我的解法

/**
 * 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 builder = "";
        queue<TreeNode*> q;
        TreeNode* temp;
        int size;

        if(root){
            q.push(root);
            while(!q.empty()){
                size = q.size();
                for(int i = 0; i < size; i++){
                    temp = q.front();
                    q.pop();
                    
                    if(temp){
                        builder += to_string(temp->val) + " ";
                        q.push(temp->left);
                        q.push(temp->right);
                    }else{
                        builder += "NULL ";
                    }
                    
                }
            }
        }

        return builder;
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        
        TreeNode* head = NULL, *temp, *n;
        if(data != ""){
            vector<string> split = split11(data, " ");
            queue<TreeNode*> q;
            head = new TreeNode(atoi(split[0].c_str()));
            q.push(head);
            int size, index = 0;

            while(!q.empty()){
                size = q.size();
                for(int i = 0; i < size; i++){
                    temp = q.front();
                    q.pop();
                    index++;
                    if(split[index] != "NULL"){
                        n = new TreeNode(atoi(split[index].c_str()));
                        temp->left = n;
                        q.push(n);
                    }

                    index++;
                    if(split[index] != "NULL"){
                        n = new TreeNode(atoi(split[index].c_str()));
                        temp->right = n;
                        q.push(n);
                    }
                }
            }
        }

        return head;
    }

    vector<string> split11(const string& in, const string& delim)
    {
        vector<string> ret;
        try
        {
            regex re{delim};
            return vector<string>{
                    sregex_token_iterator(in.begin(), in.end(), re, -1),
                    sregex_token_iterator()
            };      
        }
        catch(const std::exception& e)
        {
        }

        return ret;
    }
};

// Your Codec object will be instantiated and called as such:
// Codec codec;
// codec.deserialize(codec.serialize(root));

split11方法为似java的string.split方法，按delim分割字符串为字符串数组，由题意二叉树是按层次序列化的，所以我们序列化时也按层次序列化，注意一个不是NULL的结点，他的NULL子结点也要序列化到结果中。反序列化时，也是按层次反序列化，两层一起操作，上一层反序列化，下一层做为子结点的同时入队列，上一层反序列化完成，Index指针到了下下一层，然后下一层，下下一层进行同样的操作，还是很有技巧性的，注意一下

其它解法

/**
 * 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:
    int i;
    bool getNum(string& data,int& num){
        num = 0;
        if(data[i]=='-'){
            i++;
            while(data[i]!=','&&data[i]!=']'){
                num *= 10;
                num += data[i]-'0';
                i++;
            }
            num *= -1;
            return true;    
        }
        if(data[i]>='0'&&data[i]<='9'){
            while(data[i]!=','&&data[i]!=']'){
                num *= 10;
                num += data[i]-'0';
                i++;
            }
            return true;           
        }
        else{
            i = i+4;
            return false;
        }
    }

    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        if(!root)
            return "[]";
        string s;
        s.push_back('[');
        queue<TreeNode*> q;
        q.push(root);
        s += to_string(root->val);
        s.push_back(',');
        while(!q.empty()){
            TreeNode* son = q.front();
            q.pop();
            if(son->left){
                q.push(son->left);
                s += to_string(son->left->val);
                s.push_back(',');
            }
            else{
                s += "null,";
            }
                
            if(son->right){
                q.push(son->right);
                s += to_string(son->right->val);
                s.push_back(',');
            }
            else{
                s += "null,";
            }   
        }
        s.pop_back();
        s.push_back(']');
        return s;
    }
    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        if(data.length()==2)
            return NULL;
        queue<TreeNode*> q;
        cout << data<< endl;
        int num = 0;
        i=1;
        getNum(data,num);
        TreeNode* root=new TreeNode(num);
        q.push(root);
        TreeNode* cur;
        while(!q.empty()){
            cur = q.front();
            q.pop();
            i++;
            if(getNum(data,num)){
                TreeNode* leftson = new TreeNode(num);
                cur->left = leftson;
                q.push(leftson);
            }
            i++;
            if(getNum(data,num)){
                TreeNode* rightson = new TreeNode(num);
                cur->right = rightson;
                q.push(rightson);
            }
        } 
        return root;
    }
};

// Your Codec object will be instantiated and called as such:
// Codec codec;
// codec.deserialize(codec.serialize(root));

做法基本一样，不同的是反序列化时我用的正则split11方法，这里是用的getNum方法来获取数字或者null

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