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.

其实该题目的是设计一个序列化输出，然后反序列得到原值

你可以选择tree的各种travel， 但是要保正可以得到原来的tree

我选的是level travel

/**
* 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) {
StringBuilder arr = new StringBuilder();
arr.append("[");
if(root == null) return "[]";
ArrayList<TreeNode> queue = new ArrayList<>();
queue.add(root);
arr.append(root.val);
for (int i = 0; i < queue.size(); i++) {
TreeNode node = queue.get(i);
if(node == null) continue;
queue.add(node.left);
queue.add(node.right);
}

while(queue.get(queue.size()-1) == null){   //消去leaf的null
queue.remove(queue.size()-1);
}

for(int i = 1 ; i < queue.size(); i++){
TreeNode node = queue.get(i);
if(node == null)
arr.append(",").append("null");
else
arr.append(",").append(node.val);

}
arr.append("]");
System.out.println(arr.toString());
return arr.toString();
}

// Decodes your encoded data to tree.
public TreeNode deserialize(String data) {
if(data == null || data.length() == 0 || data.equals("[]")) return null;
String[] arr = data.substring(1, data.length() - 1).split(",");
if(arr.length == 0) return null;
ArrayList<TreeNode> list = new ArrayList<TreeNode>();
TreeNode root = new TreeNode(Integer.parseInt(arr[0]));
list.add(root);
int index = 0;
boolean isLeftNode = true;
for(int i = 1 ; i < arr.length ; i++){
if(!arr[i].equals("null")){
TreeNode node = new TreeNode(Integer.parseInt(arr[i]));
if(isLeftNode)
list.get(index).left = node;
else
list.get(index).right = node;
list.add(node);
}
if(!isLeftNode)  index++;
isLeftNode = !isLeftNode;
}
return root;
}
}

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