二叉树的序列化和反序列化

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

可以用递归，比如前序遍历，注意，因为在序列化的时候，当某节点为null,做有标记，因此，在反序列化的时候，可以仍然前序递归，但此时，字符串流的状态要随之更新。
1,2,4,#,#,#,3,5,#,#,6,#,#

/**
 * 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) {
        
        if (root==NULL)return "#";
        return to_string(root->val) + " " + serialize(root->left) + " " + serialize(root->right);
        
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        istringstream ss(data);
        return sovle(ss);
        
    }
    
    TreeNode* sovle(istringstream& ans)
    {
        string tmp;
        ans>>tmp;
        
        if (tmp=="#")return NULL;
        
        TreeNode* node = new TreeNode(stoi(tmp));
        node->left = sovle(ans);
        node->right = sovle(ans);
        return node;
        
    }
};

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

序列化的方法并非一定要前序，层序也可以，此时用到BFS和队列。

在序列化的过程中，判断一下当前是否为空，空的化就不进行push操作，不空push左右子树。

反序列化的过程依然用到了 流的特点，即根据空格分离，依次读入，并建队建树

/**
 * 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 ans;
        queue<TreeNode*>q;
        
        if (root==NULL)return ans;
        q.push(root);
        while(!q.empty())
        {
            TreeNode* tmp = q.front();
            q.pop();
            
            if (tmp)
            {
                ans += to_string(tmp->val) + " ";
                q.push(tmp->left);
                q.push(tmp->right);
                
            }
            
            else
            {
                ans += "# ";
            }

        }
        return ans;
        
        
    }

    
    
    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        
        if (data.empty())return NULL;
        
        istringstream ss(data);
        
        queue<TreeNode* >q;
        string tmp;
        ss>>tmp;
        TreeNode* node = new TreeNode(stoi(tmp));
        q.push(node);

        while(ss>>tmp)
        {
            TreeNode* node = q.front();
            q.pop();
            
            if (tmp=="#") node->left = NULL;
            else 
            {
                node->left = new TreeNode(stoi(tmp));
                q.push(node->left);
            }
            
            ss>>tmp;
            if (tmp=="#")node->right = NULL;
            else 
            {
                node->right = new TreeNode(stoi(tmp));
                q.push(node->right);
            }
            
            
        }
        
        return node;
        
    }
};

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