05 August 2008
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.
Example:
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You may serialize the following tree:
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/ \
2 3
/ \
4 5
as "[1,2,3,null,null,4,5]"
Clarification: The above format is the same as how LeetCode 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.
将一个数据结构或者对象（比如这道题的一个二叉树）转换为位序列，为序列化；同时还可以转换回来，为反序列化。位序列可以存储在内存，文件或者网络连接中。 注意不让用全局变量，成员变量和类变量，以此保持无状态。这题可以有两种解法，分别为DFS递归解法（先序+中序，或者中序+后序）和BFS非递归解法（层序）。
先序遍历的递归解法
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/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Codec {
// Encodes a tree to a single string.
public String serialize(TreeNode root) {
StringBuilder sb = new StringBuilder();
serialize(root, sb);
return sb.toString();
}
private void serialize(TreeNode root, StringBuilder sb) {
// 先序遍历序列化，如果遇到空值，填充#，在deserialize的时候根据#判断左右孩子为空的情况
if (root == null) {
sb.append("#").append(",");
} else {
sb.append(root.val).append(",");
serialize(root.left, sb);
serialize(root.right, sb);
}
}
// Decodes your encoded data to tree.
public TreeNode deserialize(String data) {
Queue<String> queue = new LinkedList<>(Arrays.asList(data.split(",")));
return deserialize(queue);
}
private TreeNode deserialize(Queue<String> queue) {
String nodeStr = queue.poll();
if (nodeStr.equals("#")) {
return null;
}
//按照serialize先序根左右的顺序挨个从队列中取出来即可
TreeNode root = new TreeNode(Integer.parseInt(nodeStr));
root.left = deserialize(queue);
root.right = deserialize(queue);
return root;
}
}
// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));
层序遍历的非递归解法
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/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Codec {
// Encodes a tree to a single string.
public String serialize(TreeNode root) {
if (root == null) return "";
Queue<TreeNode> q = new LinkedList<>();
StringBuilder res = new StringBuilder();
q.add(root);
while (!q.isEmpty()) {
TreeNode node = q.poll();
if (node == null) {
res.append("n ");
continue;
}
res.append(node.val + " ");
q.add(node.left);
q.add(node.right);
}
return res.toString();
}
// Decodes your encoded data to tree.
public TreeNode deserialize(String data) {
if (data == "") return null;
Queue<TreeNode> q = new LinkedList<>();
String[] values = data.split(" ");
TreeNode root = new TreeNode(Integer.parseInt(values[0]));
q.add(root);
for (int i = 1; i < values.length; i++) {
TreeNode parent = q.poll();
if (!values[i].equals("n")) {
TreeNode left = new TreeNode(Integer.parseInt(values[i]));
parent.left = left;
q.add(left);
}
if (!values[++i].equals("n")) {
TreeNode right = new TreeNode(Integer.parseInt(values[i]));
parent.right = right;
q.add(right);
}
}
return root;
}
}
// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));