297. Serialize and Deserialize Binary Tree

The serialization and deserialization can be done by preorder tree traversal. When I revisit the problem, I made a mistake in line 24. Without this line, there'll be "Runtime Error" when input is "null".

1:  /**  
2:   * Definition for a binary tree node.  
3:   * struct TreeNode {  
4:   *   int val;  
5:   *   TreeNode *left;  
6:   *   TreeNode *right;  
7:   *   TreeNode(int x) : val(x), left(NULL), right(NULL) {}  
8:   * };  
9:   */  
10:  class Codec {  
11:  public:  
12:    // Encodes a tree to a single string.  
13:    string serialize(TreeNode* root) {  
14:      if (root == NULL) return "#";  
15:      string s = to_string(root->val) + "," + serialize(root->left) + "," + serialize(root->right);  
16:      return s;  
17:    }  
18:    // Decodes your encoded data to tree.  
19:    TreeNode* deserialize(string data) {  
20:      return helper(data);  
21:    }  
22:    TreeNode* helper(string &data) {  
23:      if (data[0] == '#') {  
24:        if (data.size() > 1) data = data.substr(2);  
25:        return NULL;  
26:      }  
27:      TreeNode *root = new TreeNode(integer(data));  
28:      root->left = helper(data);  
29:      root->right = helper(data);  
30:      return root;  
31:    }  
32:    int integer(string &data) {  
33:      int i = data.find(',');  
34:      int val = stoi(data.substr(0, i));  
35:      data = data.substr(i+1);  
36:      return val;  
37:    }  
38:  };  
39:  // Your Codec object will be instantiated and called as such:  
40:  // Codec codec;  
41:  // codec.deserialize(codec.serialize(root));