[LeetCode] Serialize and Deserialize Binary Tree

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

1
/ \
2   3
/ \
4   5

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.

解题思路

先序遍历+递归。

实现代码

C++:

// Runtime: 140 ms
/**
* 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:
// Encodes a tree to a single string.
string serialize(TreeNode* root) {
string res = "";
serializeHelper(root, res);
return res;
}

// Decodes your encoded data to tree.
TreeNode* deserialize(string data) {
TreeNode *root = NULL;
deserializeHelper(root, data);
return root;
}

private:
void serializeHelper(TreeNode *root, string &res)
{
if (root == NULL)
{
res += "null ";
return;
}
ostringstream oss;
oss<<root->val;
res += oss.str();
res += ' ';
serializeHelper(root->left, res);
serializeHelper(root->right, res);
}

void deserializeHelper(TreeNode *&root, string& data)
{
if (data.length() == 0)
{
return;
}

string temp = data.substr(0, data.find(' '));
data = data.substr(data.find(' ') + 1);

if (temp.compare("null") == 0)
{
root = NULL;
}
else
{
root = new TreeNode(atoi(temp.c_str()));
deserializeHelper(root->left, data);
deserializeHelper(root->right, data);
}
}
};

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

Java:

// Runtime: 446 ms
/**
* 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();
return serialize(root, sb);
}

// Decodes your encoded data to tree.
public TreeNode deserialize(String data) {
StringBuilder sb = new StringBuilder(data);
TreeNode root = null;
return deserialize(root, sb);
}

private String serialize(TreeNode root, StringBuilder sb) {
if (root == null) {
sb.append("null ");
}
else {
sb.append(root.val + " ");
serialize(root.left, sb);
serialize(root.right, sb);
}

return sb.toString();
}

private TreeNode deserialize(TreeNode root, StringBuilder sb) {
if (sb.length() == 0) {
return null;
}

String temp = sb.substring(0, sb.indexOf(" "));
sb = sb.delete(0, sb.indexOf(" ") + 1);
if (temp.equals("null")) {
root = null;
}
else {
root = new TreeNode(Integer.parseInt(temp));
root.left = deserialize(root.left, sb);
root.right = deserialize(root.right, sb);
}

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