[LeetCode 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.

solution:

1. preorder traversal, use another helper class(StringToken) to split string, and record current position

2. level order traversal

public class Codec {

// Encodes a tree to a single string.
public String serialize(TreeNode root) {
StringBuilder sb = new StringBuilder();
helptoserialize(root, sb);
return sb.toString();
}

public void helptoserialize(TreeNode root, StringBuilder sb) {
if(root == null) {
sb.append("# ");
} else {
sb.append(root.val);
sb.append(" ");
helptoserialize(root.left, sb);
helptoserialize(root.right, sb);
}
}

// Decodes your encoded data to tree.
public TreeNode deserialize(String data) {
TreeNode root = helptodeserialize(new StringToken(data, " "));
return root;
}

public TreeNode helptodeserialize(StringToken token) {
if(!token.hasMoreElement()) return null;
String t1 = token.nextElement();
if(t1.equals("#")) return null;
TreeNode root = new TreeNode(Integer.valueOf(t1));
root.left = helptodeserialize(token);
root.right = helptodeserialize(token);
return root;
}

class StringToken {
public int len = 0;
ArrayList<String> dataStrings = new ArrayList<>();
public StringToken (String data, String spliter) {
String[] datas = data.split(spliter);
for(int i=0;i<datas.length;i++) {
dataStrings.add(datas[i]);
}
}
public boolean hasMoreElement() {
return len<dataStrings.size();
}
public String nextElement() {
return dataStrings.get(len++);
}
}
}