leetcode 297

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.

Example: 

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You may serialize the following tree:

    1
   / \
  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.

分析

使用先序遍历依次输出节点，并以 , 进行分割，遇到NULL也要进行记录以表示要进行回溯。这里NULL可以使用X进行表示。

代码

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/**
 * 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) {
            ans += ",X";
            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) {
        string ans;
        serialize(root, ans);
        return ans.substr(1);
    }

    TreeNode *deserialize(queue<string> &q)  {
        if (q.size() == 0)
            return NULL;
        string t = q.front();
        q.pop();
        if (t == "X") return NULL;
        TreeNode *node = new TreeNode(stoi(t));
        node->left = deserialize(q);
        node->right = deserialize(q);
        return node;
    }
    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        stringstream input(data);
        string t;
        queue<string> q;
        while (getline(input, t, ',')) {
            q.push(t);
        }
        return deserialize(q);
    }
};

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

相似题目：https://leetcode.com/problems/serialize-and-deserialize-bst/