297. Serialize and Deserialize Binary Tree

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:
就是pre-order ，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

/**
 * 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) {
        String res = "";
        if(root == null){
            return "#,";
        }else{
            res += "" + root.val + "," ;
            res += serialize(root.left);
            res += serialize(root.right);
        }
        return res;
    }

    // Decodes your encoded data to tree.
    String str = "";
    public TreeNode deserialize(String data) {
        str = data;
        return helper();
    }
    private TreeNode helper(){
        if(str.length()==0)
            return null;
        if(str.charAt(0)=='#'){
            str= str.substring(2);
            return null;
        }
        int iComma = str.indexOf(",");
        int rootVal = Integer.parseInt(str.substring(0,iComma));
        TreeNode root = new TreeNode(rootVal);
        str = str.substring(iComma+1);
        root.left = helper();
        root.right = helper();
        return root;
    }
    
}

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

Mistakes:

1. 加成功一个root.value之后，忘了加逗号