[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.

Credits:

Special thanks to @Louis1992 for adding this problem and creating all test cases

basic idea

1) pre order/post order sequence with in order sequence can construct binary tree

    we can store tree's pre/post order and in order sequence ,and reconstruct based on that

    preorder#inorder   or postorder#inorder

2) level order sequence

    BFS solution

/**
 * 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:
    /*algorithm
        regard tree as full binary tree
        so i's child is 2i and 2i+1
    */
    
    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        queue<TreeNode*>Q;
        string s;
        if(root)Q.push(root);
        while(!Q.empty()){
            int size = Q.size();
            for(int i = 0;i < size;i++){
                TreeNode* t = Q.front();Q.pop();
                if(s.size() > 0)s += ",";
                s += t?to_string(t->val):"#";
                if(t){
                    Q.push(t->left);
                    Q.push(t->right);
                }
            }
        }
        return s;
    }
    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        if(data.size() < 1) return NULL;
        queue<TreeNode*>Q;
        int i = 0;
        vector<string>vec;
        TreeNode* root;
        while(i < data.size()){
            int j = i;
            while(j < data.size()&&data[j] != ',')++j;
            vec.push_back(data.substr(i,j-i));
            i = j+1;
        }
        root = new TreeNode(atoi(vec[0].c_str()));
        Q.push(root);
        i = 1;
        while(!Q.empty() && i < vec.size()){
            int size = Q.size();
            for(int j = 0;j < size;j++){
                TreeNode* t = Q.front();Q.pop();
                if(vec[i] != "#"){
                    t->left = new TreeNode(atoi(vec[i].c_str()));
                    Q.push(t->left);
                }
                ++i;
                if(vec[i] != "#"){
                    t->right = new TreeNode(atoi(vec[i].c_str()));
                    Q.push(t->right);
                }
                ++i;
            }
        }
        return root;
    }
};

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