leetcode 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.

按照剑指offer的方法修改的代码：

序列化：用前序遍历来递归遍历树，存进string。

反序列化：也是前序遍历的方式来递归生成树。

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)
{
vt.clear();
serialize_help(root);
string ret = "";
for (auto it : vt)
ret += it + "#";
return ret;
}

void serialize_help(TreeNode* root)
{
if (!root)
vt.push_back("$");
else
{
vt.push_back(to_string(root->val));
serialize_help(root->left);
serialize_help(root->right);
}
}

// Decodes your encoded data to tree.
TreeNode* deserialize(string data)
{
vt.clear();
while (1)
{
int pos = data.find('#');
if (pos < 0 || pos >= data.size())
break;
vt.push_back(data.substr(0, pos));
data = data.substr(pos + 1);
}

makepos = -1;
return deserialize_help();
}

TreeNode * deserialize_help()
{
makepos++;
if (vt[makepos] == "$")
return nullptr;

TreeNode* root = new TreeNode(atoi(vt[makepos].c_str()));
root->left = deserialize_help();
root->right = deserialize_help();
return root;
}
private:
vector<string> vt; //写为成员变量完全是为了方便，也可以写成普通变量，然后引用传递给递归的函数。
int makepos; //也可以不用这个。每次遍历vt第一个，然后删除第一个。
};