DFS比BFS实现起来更简洁。

序列化格式：

    8
   / \
  12  2
     / \
    6   4
"8 12 null null 2 6 null null 4 null null"

DFS实现 - 先序遍历：

void ser(TreeNode *p, ostringstream &oss) {
  if (!p) {
    oss << "null ";
    return;
  }

  oss << p->val << " ";
  ser(p->left, oss);
  ser(p->right, oss);
}

void des(TreeNode **p, istringstream &iss) {
  string s;
  iss >> s;
  if (s == "null")
    return;

  *p = new TreeNode(atoi(s.c_str()));
  des(&((*p)->left), iss);
  des(&((*p)->right), iss);
}

class Solution {
public:
  // Encodes a tree to a single string.
  string serialize(TreeNode* root) {
    ostringstream oss;
    ser(root, oss);
    return oss.str();
  }

  // Decodes your encoded data to tree.
  TreeNode* deserialize(string data) {
    TreeNode *root;
    istringstream iss(data);
    des(&root, iss);
    return root;
  }
};