序列化和反序列化二叉树 Serialize and Deserialize Binary Tree

问题：

Serialization is the process of converting a data structure or object into a sequence of bits so that it can be stored in a file or memory buffer, or transmitted across a network connection link to be reconstructed later in the same or another computer environment.

Design an algorithm to serialize and deserialize a binary tree. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that a binary tree can be serialized to a string and this string can be deserialized to the original tree structure.

For example, you may serialize the following tree

as "[1,2,3,null,null,4,5]", just the same as how LeetCode OJ serializes a binary tree. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.

Note: Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.

解决：

① 理论上说所有遍历的方法都可以。

但是为了使serialize和deserialize的过程都尽量最简单，preorder是不错的选择。serialize的话，dfs比较好写，deserialize的话preorder和bfs比较好写。用“,”作为分隔符，“#”来表示null。例子里serialize之后结果就变成"1,2,3,#,#,4,5"。deserialize的时候用一个queue来保存string。

Time: O(N), Space: O(N)

public class Codec { //25ms
// Encodes a tree to a single string.
public String serialize(TreeNode root) {
if (root == null) return "";
StringBuilder encodedStr = new StringBuilder();
encode(root,encodedStr);
return encodedStr.substring(1).toString();
}
public void encode(TreeNode root,StringBuilder sb){
if (root == null){
sb.append(",#");
return;
}
sb.append(",").append(root.val);
encode(root.left,sb);
encode(root.right,sb);
}
// Decodes your encoded data to tree.
public TreeNode deserialize(String data) {
if (data.length() == 0) return null;
Queue<String> queue = new LinkedList<>(Arrays.asList(data.split(",")));
return decode(queue);
}
public TreeNode decode(Queue<String> queue){
if (queue.isEmpty()) return null;
String cur = queue.poll();
if (cur.equals("#")) return null;
TreeNode root = new TreeNode(Integer.valueOf(cur));
root.left = decode(queue);
root.right = decode(queue);
return root;
}
}
// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));

来源：oschina

链接：https://my.oschina.net/u/2968041/blog/1602614

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A Tree

Sequence