297. Serialize and Deserialize Binary Tree (Hard)

Q:

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.

A:
pre-order 遍历  ，tricky的地方就是，每次加入一个数字（或者#)之后，后面都要加上 个逗号
被problem description所影响了。 

人家用的是layer level。 我自己选的用递归，

/**

* 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) {

if(!root)

return "#";// for nullptr

string res = to_string(root->val);

res += "," + serialize(root->left);

res += "," + serialize(root->right);

cout << res << endl;

return res;

}

// Decodes your encoded data to tree.

TreeNode* deserialize(string data) {

stringstream ss(data);

string item;

vector<TreeNode*> V;

while (getline(ss, item, ',')) {

V.push_back(item == "#"? nullptr : new TreeNode(stoi(item)));

}

int start = 0;

return helper(V, start);

}

private:

TreeNode* helper(vector<TreeNode*> &V, int& start){

if(start >= V.size()){

return nullptr;

}

auto root = V[start];

if(root){

start++;

root->left = helper(V, start);// here the start can NOT be written as start++, nor ++start

start++;

root->right = helper(V, start);

}

return root;

}

};

// Your Codec object will be instantiated and called as such:

// Codec ser, deser;

// TreeNode* ans = deser.deserialize(ser.serialize(root));

Mistakes:

1. 加成功一个root.value之后，忘了加逗号