序列化格式：

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

BFS实现 - 层级遍历：

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

    // bfs
    ostringstream oss;
    queue<TreeNode*> que;
    que.push(root);
    while (!que.empty()) {
      TreeNode* p = que.front();
      que.pop();
      if (p) {
        oss << p->val << " ";
        que.push(p->left);
        que.push(p->right);
      } else {
        oss << "null" << " ";
      }
    }
    return oss.str();
  }

  // Decodes your encoded data to tree.
  TreeNode* deserialize(string data) {
    istringstream iss(data);

    string s;
    iss >> s;
    if (s == "null")
      return NULL;

    // init root
    TreeNode *root = new TreeNode(atoi(s.c_str()));
    queue<TreeNode*> que;
    que.push(root);

    // bfs
    while (!que.empty()) {
      TreeNode* p = que.front();
      que.pop();

      string l, r;
      iss >> l >> r;
      if (l != "null") {
        p->left = new TreeNode(atoi(l.c_str()));
        que.push(p->left);
      }
      if (r != "null") {
        p->right = new TreeNode(atoi(r.c_str()));
        que.push(p->right);
      }
    }
    return root;
  }
};