LeetCode 297. 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:
    
    string find(string s,int pos)
    {
        string str="";
        for(int i=pos;i<s.length();i++)
        {
            if(s[i]==',')
                break;
            str +=s[i];
        }
        return str;
    }

    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        
        string s="";
         fun2(root,s);
        return s;
        
    }
    
    void fun2(TreeNode* root,string& res)
    {
        if(root!=NULL)
        {
            res+=to_string(root->val);
            res+=',';
            fun2(root->left,res);
            res+=',';
            fun2(root->right,res);
        }
        else
        {
            res+='.';
        }
        
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        
        TreeNode* root=NULL;
        fun(data,root,0);
        
        return root;
        
    }
    
    int fun(string data,TreeNode* &tree,int pos)
    {
        string s = find(data,pos);
        if(s[0]=='.')
        {
            tree=NULL;
            return pos+s.length()+1;
        }
        else
        {
            tree = new TreeNode(atoi(s.c_str()));
            int x = fun(data,tree->left,pos+s.length()+1);
            int y = fun(data,tree->right,x);
            
            return y;
        }
    }         
};

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