297. Serialize and Deserialize Binary Tree

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.

Subscribe to see which companies asked this question.

二叉树的序列化和反序列化。这里都用了广度优先的方式进行，用队列实现。实现没什么难点，注意空指针就行了。

这里遇到了坑，在序列化的时候，想着一个节点用完了，后面就没有用处了，反正后面返回的是字符串，所以每个节点的内存都释放掉了。

结果通过不了，内存出错了。可能测试的时候会出现一颗二叉树用几次的情况吧。

代码：

class Codec {public:// Encodes a tree to a single string.string serialize(TreeNode* root) {string res;if(!root) return "null";queue<TreeNode*>q;q.push(root);while(!q.empty()){TreeNode* tmp = q.front();q.pop();if(!tmp){res += "null,";}else{res += to_string(tmp->val) + ",";q.push(tmp->left);q.push(tmp->right);}}return res;}// Decodes your encoded data to tree.TreeNode* deserialize(string data) {int pos = 0, n = data.size();data += ",";TreeNode* root = build_node(data, pos);if(!root) return NULL;queue<TreeNode*>q;q.push(root);while(!q.empty()){TreeNode* tmp = q.front();q.pop();if(pos >= n) break;tmp->left = build_node(data, pos);if(tmp->left) q.push(tmp->left);if(pos >= n) break;tmp->right = build_node(data, pos);if(tmp->right) q.push(tmp->right);}return root;}private:TreeNode* build_node(const string& s, int& pos){int end = s.find(",", pos);string tmp = s.substr(pos, end-pos);pos = end + 1;if(tmp == "null"){return NULL; }else{TreeNode *node = new TreeNode(stoi(tmp));return node;}}};