Leetcode192: 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.

/**
* 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 result;
traverse(root, result);
return result;
}

// Decodes your encoded data to tree.
TreeNode* deserialize(string data) {
int start = 0;
return helper(data, start);
}

private:
void traverse(TreeNode* root, string& result) {
if(result.size() != 0) {
result.push_back(',');
}
if(root == NULL) {
result += "NULL";
return;
}
result += to_string(root -> val);
traverse(root -> left, result);
traverse(root -> right, result);
}

TreeNode* helper(string data, int& start) {
if(start >= data.size()) {
return NULL;
}
int index = data.find_first_of(',', start);
if(index == string::npos) {
index = data.size();
}
string tmp = data.substr(start, index - start);
start = index + 1;
if(tmp == "NULL") {
return NULL;
}
TreeNode* root = new TreeNode(stoi(tmp));
root -> left = helper(data, start);
root -> right = helper(data, start);
return root;
}
};

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