[LeetCode] Serialize and Deserialize Binary Tree

I adopt a way similar to that of the OJ to serialize the binary tree. For the following tree, my serialization result is "1,2,3,null,null,4,5," (note the last comma).

1
/ \
2   3
/ \
4   5

The serialization and deserialization both use the BFS traversal. The code is as follows.

/**
* 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) {
vector<string> nodes;
queue<TreeNode*> level;
level.push(root);
while (!level.empty()) {
int n = level.size();
for (int i = 0; i < n; i++) {
TreeNode* node = level.front(); level.pop();
nodes.push_back(node ? to_string(node->val) + "," : "null,");
if (node) {
level.push(node->left);
level.push(node->right);
}
}
}
while (!nodes.empty() && nodes.back() == "null,")
nodes.pop_back();
string tree;
for (string node : nodes) tree += node;
return tree;
}

// Decodes your encoded data to tree.
TreeNode* deserialize(string data) {
if (data.empty()) return NULL;
stringstream ss(data);
string vs;
getline(ss, vs, ',');
TreeNode* root = new TreeNode(stoi(vs));
queue<TreeNode*> level;
level.push(root);
while (!level.empty()) {
int n = level.size();
for (int i = 0; i < n; i++) {
TreeNode* node = level.front(); level.pop();
if (getline(ss, vs, ',') && vs != "null") {
node->left = new TreeNode(stoi(vs));
level.push(node -> left);
}
if (getline(ss, vs, ',') && vs != "null") {
node->right = new TreeNode(stoi(vs));
level.push(node -> right);
}
}
}
return root;
}
};

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