449 Serialize and Deserialize BST 序列化和反序列化二叉搜索树
Description:
Serialization is 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 search tree. There is no restriction on how your serialization/deserialization algorithm should work. You need to ensure that a binary search tree can be serialized to a string, and this string can be deserialized to the original tree structure.
The encoded string should be as compact as possible.
Example:
Example 1:
Input: root = [2,1,3]
Output: [2,1,3]
Example 2:
Input: root = []
Output: []
Constraints:
The number of nodes in the tree is in the range [0, 10^4].
0 <= Node.val <= 10^4
The input tree is guaranteed to be a binary search tree.
题目描述:
序列化是将数据结构或对象转换为一系列位的过程，以便它可以存储在文件或内存缓冲区中，或通过网络连接链路传输，以便稍后在同一个或另一个计算机环境中重建。
设计一个算法来序列化和反序列化 二叉搜索树 。 对序列化/反序列化算法的工作方式没有限制。 您只需确保二叉搜索树可以序列化为字符串，并且可以将该字符串反序列化为最初的二叉搜索树。
编码的字符串应尽可能紧凑。
示例 :
示例 1：
输入：root = [2,1,3]
输出：[2,1,3]
示例 2：
输入：root = []
输出：[]
提示：
树中节点数范围是 [0, 10^4]
0 <= Node.val <= 10^4
题目数据 保证 输入的树是一棵二叉搜索树。
注意：
不要使用类成员/全局/静态变量来存储状态。 你的序列化和反序列化算法应该是无状态的。
思路:
序列化采用后序遍历, 这样反序列化时可以直接从列表的最后拿不需要记录节点下标
时间复杂度O(n), 空间复杂度O(n)
代码:
C++:
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Codec
{
private:
void post_order(TreeNode *root, string &result)
{
if (!root) return;
post_order(root -> left, result);
post_order(root -> right, result);
result += to_string(root -> val) + " ";
}
TreeNode* helper(int lower, int upper, vector &nums)
{
if (nums.empty()) return nullptr;
int val = nums.back();
if (val < lower or val > upper) return nullptr;
nums.pop_back();
TreeNode *root = new TreeNode(val);
root -> right = helper(val, upper, nums);
root -> left = helper(lower, val, nums);
return root;
}
public:
// Encodes a tree to a single string.
string serialize(TreeNode* root)
{
string result;
post_order(root, result);
return result;
}
// Decodes your encoded data to tree.
TreeNode* deserialize(string data)
{
if (data.empty()) return nullptr;
stringstream ss;
string num;
vector nums;
ss << data;
while (ss >> num) nums.emplace_back(stoi(num));
return helper(INT_MIN, INT_MAX, nums);
}
};
// Your Codec object will be instantiated and called as such:
// Codec* ser = new Codec();
// Codec* deser = new Codec();
// string tree = ser->serialize(root);
// TreeNode* ans = deser->deserialize(tree);
// return ans;
Java:
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Codec {
private StringBuilder postorder(TreeNode root, StringBuilder sb) {
if (root == null) return sb;
postorder(root.left, sb);
postorder(root.right, sb);
sb.append(root.val);
sb.append(' ');
return sb;
}
// Encodes a tree to a single string.
public String serialize(TreeNode root) {
StringBuilder sb = postorder(root, new StringBuilder());
if (sb.length() > 0) sb.deleteCharAt(sb.length() - 1);
return sb.toString();
}
public TreeNode helper(Integer lower, Integer upper, ArrayDeque nums) {
if (nums.isEmpty()) return null;
int val = nums.getLast();
if (val < lower || val > upper) return null;
nums.removeLast();
TreeNode root = new TreeNode(val);
root.right = helper(val, upper, nums);
root.left = helper(lower, val, nums);
return root;
}
// Decodes your encoded data to tree.
public TreeNode deserialize(String data) {
if (data.isEmpty()) return null;
ArrayDeque nums = new ArrayDeque();
for (String s : data.split("\\s+")) nums.add(Integer.valueOf(s));
return helper(Integer.MIN_VALUE, Integer.MAX_VALUE, nums);
}
}
// Your Codec object will be instantiated and called as such:
// Codec ser = new Codec();
// Codec deser = new Codec();
// String tree = ser.serialize(root);
// TreeNode ans = deser.deserialize(tree);
// return ans;
Python:
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Codec:
def serialize(self, root: TreeNode) -> str:
"""Encodes a tree to a single string.
"""
def postorder(root: TreeNode) -> List[int]:
return postorder(root.left) + postorder(root.right) + [root.val] if root else []
return ' '.join(map(str, postorder(root)))
def deserialize(self, data: str) -> TreeNode:
"""Decodes your encoded data to tree.
"""
def helper(lower:int = float('-inf'), upper:int = float('inf')) -> TreeNode:
if not data or data[-1] < lower or data[-1] > upper:
return None
val = data.pop()
root, root.right, root.left = TreeNode(val), helper(val, upper), helper(lower, val)
return root
data = [int(x) for x in data.split(' ') if x]
return helper()
# Your Codec object will be instantiated and called as such:
# Your Codec object will be instantiated and called as such:
# ser = Codec()
# deser = Codec()
# tree = ser.serialize(root)
# ans = deser.deserialize(tree)
# return ans