本题虽然是Hard，但是难度其实和Medium相仿，只是之前没见过类似的题目，就是无论是serialize还是deseialize，都用很朴素的recursion即可解出。

    • 时间复杂度:所有node全都遍历一遍，所以是297. Serialize and Deserialize Binary Tree[没有解出] - 图1

    直接看代码，没有太多神奇的地方:

    1. /**
    2.  * Definition for a binary tree node.
    3.  * public class TreeNode {
    4.  *     int val;
    5.  *     TreeNode left;
    6.  *     TreeNode right;
    7.  *     TreeNode(int x) { val = x; }
    8.  * }
    9.  */
    10. public class Codec {
    11.   // Encodes a tree to a single string.
    12.   public String serialize(TreeNode root) {
    13.     StringBuilder sb = new StringBuilder();
    14.     buildString(sb, root);
    15.     return sb.toString();
    16.   }
    17.   // Decodes your encoded data to tree.
    18.   public TreeNode deserialize(String data) {
    19.     return buildTree(new LinkedList<>(Arrays.asList(data.split(","))));
    20.   }
    22.   private void buildString(StringBuilder sb, TreeNode node) {
    23.     if (node == null) {
    24.       sb.append("#");
    25.       sb.append(",");
    26.     }
    27.     else {
    28.       sb.append(String.valueOf(node.val));
    29.       sb.append(",");
    30.       buildString(sb, node.left);
    31.       buildString(sb, node.right);
    32.     }
    33.   }
    35.   private TreeNode buildTree(Queue<String> queue) {
    36.     String value = queue.poll();
    37.     if ("#".equals(value)) {
    38.       return null;
    39.     }
    40.     TreeNode node = new TreeNode(Integer.valueOf(value));
    41.     node.left = buildTree(queue);
    42.     node.right = buildTree(queue);
    43.     return node;
    44.   }
    45. }
    46. // Your Codec object will be instantiated and called as such:
    47. // Codec codec = new Codec();
    48. // codec.deserialize(codec.serialize(root));