297. 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.

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 思路：

    采用前序遍历的方法。在序列化的时候，遇到空节点，返回的字符串加上NULL，遇到非空节点，返回的字符串加上节点的值。递归得到最终的序列字符串。保证在节点之间由逗号隔开。

    在反序列化的时候，指明从start位置开始寻找逗号的位置，这样就可以截取节点对应的字符串。遇到“NULL”，直接返回NULL。否则根据字符串的值建立根节点，递归建立左右子树。

/**
 * 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 result ="";
        preorder(root,result);
        return result;
    }
    void preorder(TreeNode *root ,string &result){
        if(result.length()>0)
            result.push_back(',');
        if(!root){
            result+="NULL";
            return;
        }
        result+=to_string(root->val);
        preorder(root->left,result);
        preorder(root->right,result);
    }
    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        int start=0;
        return de_preorder(data,start);
    }
    TreeNode *de_preorder(string&data,int &start){
        if(start>=data.length())
            return NULL;
        int index =data.find(',',start);
        if(index ==string::npos){
            index =data.length();
        }
        string tmp =data.substr(start,index -start);
        start=index+1;
        if(tmp=="NULL"){
            return NULL;
        }
        TreeNode *root =new TreeNode (stoi(tmp));
        root->left =de_preorder(data,start);
        root->right=de_preorder(data,start);
        return root;
    }
};
// Your Codec object will be instantiated and called as such:
// Codec codec;
// codec.deserialize(codec.serialize(root));