Serialize and Deserialize Binary Tree

1) Please remember how to split a string by char;

code

/**
 * 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 ret;
        if(root == NULL)
        {
            return ret;
        }

        list<TreeNode*> queue;
        queue.push_back(root);
        bool hasNextLevel = true;

        while(!queue.empty() && hasNextLevel)
        {
            int size = queue.size();
            hasNextLevel = false;
            for(int i = 0; i<size; i++)
            {
                TreeNode* p = queue.front();
                queue.pop_front();

                if(p == NULL)
                {
                    ret += "null,";
                    continue;
                }

                ret += to_string(p->val) + ",";
                queue.push_back(p->left);
                queue.push_back(p->right);
                if(p->left != NULL || p->right != NULL)
                {
                    hasNextLevel = true;
                }
            }
        }

        ret.pop_back();
        return ret;
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        //cout<<data<<endl;
        if(data.empty())
        {
            return NULL;
        }

        vector<string> vec = split(data, ',');

        TreeNode* root = new TreeNode(stoi(vec[0]));
        TreeNode*p = root;
        list<TreeNode*> queue;


        for(int i = 1; i<vec.size(); i++)
        {
            if(i%2 == 1)
            {
                if(vec[i] != "null")
                {
                    TreeNode* newnode = new TreeNode(stoi(vec[i]));
                    p->left = newnode;
                    queue.push_back(newnode);
                }
            }
            else
            {
                if(vec[i] != "null")
                {
                    TreeNode* newnode = new TreeNode(stoi(vec[i]));
                    p->right = newnode;
                    queue.push_back(newnode);
                }
                p = queue.front();
                queue.pop_front();
            }
        }

        return root;
    }

    vector<string> split(const string& str, char delim)
    {
        vector<string> ret;
        stringstream ss;
        ss.str(str);
        string item;
        while(getline(ss, item, delim))
        {
            ret.push_back(item);
        }
        return ret;
    }
};

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