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.

I currently only wrote a recursive way.  I will update with a iterative way.

#include <string>
#include <iostream>
#include <sstream>
using namespace std;

struct TreeNode {
  int val;
  TreeNode* left;
  TreeNode* right;
  TreeNode(int value) : val(value), left(NULL), right(NULL) {}
};

/*
  The tree is set up as 
       1
    2    3
  N  N  N  N
*/

TreeNode* setUpTree() {
  TreeNode* root = new TreeNode(1);
  root->left = new TreeNode(2);
  root->right = new TreeNode(3);
  return root;
}

void serializeTree(TreeNode* root, string& output) {
  if(root == NULL) {output += "#"; return;}
  else {
    output += to_string(root->val);
    serializeTree(root->left, output);
    serializeTree(root->right, output);
  }
}
void deserialize_root(TreeNode*& root, istringstream& in) {
  char val;
  in >> val;
  if(val != '#') {
    root = new TreeNode(val - '0');
    deserialize_root(root->left, in);
    deserialize_root(root->right, in);
  }
}

void deserialize(TreeNode*& root, string input) {
  int pos = 0;
  istringstream in(input);
  deserialize_root(root, in);
}

int main(void) {
  TreeNode* root = setUpTree();
  string output = "";
  serializeTree(root, output);
  cout << output << endl;

  TreeNode* newRoot = NULL;
  string newOutput = "";
  deserialize(newRoot, output);
  serializeTree(newRoot, newOutput);

  cout << newOutput << endl;
}