[LeetCode] Serialize and Deserialize Binary Tree

Question can be found
https://leetcode.com/problems/serialize-and-deserialize-binary-tree/

/**
 * 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) {
        if(root == NULL) return "";
        vector<string> v;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()){
            TreeNode *t = q.front();
            q.pop();
            if(t == NULL) {
                v.push_back("null");
                continue;
            }
            
            v.push_back(to_string(t->val));
            
            q.push(t->left);
            q.push(t->right);
        }
        
        int l = v.size() - 1;
        while(v[l] == "null") l--;
        
        string res = v[0];
        for(int i=1;i<=l;i++){
            res += "," + v[i];
        }
        return res;
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        if(0 == data.length()) return NULL;
        vector<string> v;
        int start = 0;
        for(int i=1;i<data.length();i++) {
            if(data[i] != ',') continue;
            v.push_back(data.substr(start, i-start));
            start = i+1;
        }
        v.push_back(data.substr(start));
        int index = 0;
        
        TreeNode *node = new TreeNode(stoi(v[index]));
        
        queue<TreeNode*> nodesOnLevel;
        int count = v.size();
        nodesOnLevel.push(node);
        
        while(!nodesOnLevel.empty()) {
            int size = nodesOnLevel.size();
            for(int i=0;i<size;i++){
                
                int left = index + 1;
                if(left >= count) return node;
                if(v[left] != "null") {
                    TreeNode *cur = new TreeNode(stoi(v[left]));
                    nodesOnLevel.front()->left = cur;
                    nodesOnLevel.push(cur);
                }
                
                int right = index + 2;
                if(right >= count) return node;
                if(v[right] != "null") {
                    TreeNode *cur = new TreeNode(stoi(v[right]));
                    nodesOnLevel.front()->right = cur;
                    nodesOnLevel.push(cur);
                }
                nodesOnLevel.pop();
                index += 2;
            }
        }
        
        return node;
    }
};

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