serialize and deserialize

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */


std::vector<std::string> split(const std::string& str, char delim) {
  std::vector<std::string> tokens;
  size_t start = 0;
  size_t end = 0;
  while ((end = str.find(delim, start)) != std::string::npos) {
    tokens.push_back(str.substr(start, end - start));
    start = end + 1;
  }
  tokens.push_back(str.substr(start));
  return tokens;
}
class Codec {
public:
    int ind;
    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        if(!root) return "N";
        return
            to_string(root->val)+","+serialize(root->left)+","
            +serialize(root->right);
    }
    TreeNode* construct(vector<string> data){
        string res=data[ind];
        if(data[ind]=="N") {
            ind+=1;
            return NULL;

        }
        int val=stoi(res);
        ind+=1;//index has to be incremented only after it is accessed
        TreeNode* curr=new TreeNode(val);
        curr->left=construct(data);
        curr->right=construct(data);
        return curr;

    }
    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        ind=0;
        vector<string> arr=split(data,',');
        return construct(arr);
        return NULL;
    }
};

// Your Codec object will be instantiated and called as such:
// Codec ser, deser;
// TreeNode* ans = deser.deserialize(ser.serialize(root));