算法1： DFS

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

  // Decodes your encoded data to tree.
  TreeNode* deserialize(string data) {
    if (data.empty()) return nullptr;
    istringstream in(data);
    return deserialize(in);
  }

private:
  void serialize(TreeNode* root, ostringstream& out) {
    if (!root) {
      out << "# ";
      return;
    }
    out << to_string(root->val) << ' ';
    serialize(root->left, out);
    serialize(root->right, out);
  }

  TreeNode* deserialize(istringstream& in) {
    string val;
    in >> val;
    if (val == "#") return nullptr;

    auto root = new TreeNode(stoi(val));
    root->left  = deserialize(in);
    root->right = deserialize(in);
    return root;
  }
};

TODO： 算法2 BFS