leetcode 449

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 search tree. There is no restriction on how your serialization/deserialization algorithm should work. You just 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.

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

分析

树的序列化问题的关键在于重新建立这棵树的时候什么时候进行回溯，在Serialize and Deserialize Binary Tree中，我们可以使用在先序遍历遇到NULL时增加节点来确定回溯的条件。而在BST中，就不需要那么做了。因为我们可以对一颗子树中所有的值设定上限和下限来确定回溯的条件。

代码

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
/**
 * 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 {
public:
    void serialize(TreeNode *root, string &ans) {
        if (root==NULL)
            return;
        ans += "," + to_string(root->val);
        serialize(root->left,  ans);
        serialize(root->right, ans);
    }
    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        if (root == NULL) return "null";
        string ans;
        serialize(root, ans);
        return ans.substr(1);
    }
    TreeNode *deserialize(queue<int> &q, int lo, int hi) {
        if (q.size() == 0)
            return NULL;
        if (q.front() < lo || q.front() > hi)
            return NULL;
        TreeNode *node = new TreeNode(q.front());
        q.pop();
        node->left  = deserialize(q, lo, node->val);
        node->right = deserialize(q, node->val, hi);
        return node;
    }
    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        if (data == "null")
            return NULL;
        stringstream input(data);
        string t;
        queue<int> q;
        while (getline(input, t, ',')) {
            q.push(stoi(t));
        }
        return deserialize(q, INT_MIN, INT_MAX);
    }
};

// Your Codec object will be instantiated and called as such:
// Codec codec;
// codec.deserialize(codec.serialize(root));