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.

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这种serialization的方法很好看，用的是preOrder， 用途很广，做法也是挺漂亮的

1. cc150里面的找一个超大的树A是不是另一个超大的树B的子树，也可以先压扁，然后如果一个树的序列是另外一个数序列的子串，那么就是子树

2. snapchat有个面经，“是从binary tree里找出所有duplicate的树。return应该是一个key value pair，key是任何形式表示的树，value是这个树出现的次数。我的大致思路就是对每个node，做出以这个node为根的树的serialization。然后以这个serialization为key扔到hashtable里就可以。followup就是问时间和空间复杂度。”也是可以把每一个node压扁了，然后用hashSet统计出现的次数

 

/**
 * 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 sb = new StringBuilder();
        getSerialization(root, sb);
        return sb.toString();
    }
    
    private void getSerialization(TreeNode root, StringBuilder sb) {
        if(root == null) {
            sb.append("#").append(",");
            return;
        }
        sb.append(root.val + ",");
        getSerialization(root.left, sb);
        getSerialization(root.right, sb);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        String[] nodes = data.split(",");
        Queue<String> queue = new LinkedList<>();
        queue.addAll(Arrays.asList(nodes));
        return getTree(queue);
    }
    
    private TreeNode getTree(Queue<String> queue) {
        String cur = queue.poll();
        TreeNode root = null;
        if(cur.equals("#")) {
            return root;
        } else {
            root = new TreeNode(Integer.parseInt(cur));
            root.left = getTree(queue);
            root.right = getTree(queue);
        }
        return root;
    }
}

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