# 0297. Serialize and Deserialize Binary Tree

<https://leetcode.com/problems/serialize-and-deserialize-binary-tree>

## Description

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.

**Clarification:** The input/output format is the same as [how LeetCode serializes a binary tree](https://github.com/jaywinhuang/leetcode/blob/master/faq/README.md#binary-tree). You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/09/15/serdeser.jpg)

```
**Input:** root = [1,2,3,null,null,4,5]
**Output:** [1,2,3,null,null,4,5]
```

**Example 2:**

```
**Input:** root = []
**Output:** []
```

**Example 3:**

```
**Input:** root = [1]
**Output:** [1]
```

**Example 4:**

```
**Input:** root = [1,2]
**Output:** [1,2]
```

**Constraints:**

* The number of nodes in the tree is in the range `[0, 104]`.
* `-1000 <= Node.val <= 1000`

## ac1: BFS

```java
/**
 * 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) {
        // edge cases
        if (root == null) return "null";

        StringBuilder sb = new StringBuilder();
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        while (!q.isEmpty()) {
            TreeNode h = q.poll();
            if (h == null) {
                sb.append("null,");
            } else {
                sb.append("" + h.val + ",");
                q.offer(h.left);
                q.offer(h.right);
            }
        }

        sb.setLength(sb.length()-1); // remove last ","
        return sb.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        String[] s = data.split(",");
        if ("null".equals(s[0])) return null;
        TreeNode root = new TreeNode(Integer.parseInt(s[0]));  // set root

        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        int i = 1;
        while(!q.isEmpty()) {
            TreeNode h = q.poll();
            String l = s[i++];
            h.left = "null".equals(l) ? null : new TreeNode(Integer.parseInt(l));
            String r = s[i++];
            h.right = "null".equals(r) ? null : new TreeNode(Integer.parseInt(r));
            if (h.left != null) q.offer(h.left);
            if (h.right != null) q.offer(h.right);
        }

        return root;
    }
}

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

/*
1. BFS
2. DFS, pre-order
*/
```

## ac2: DFS pre-order

```java
/**
 * 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();
        seri(root, sb);
        return sb.toString();
    }
    private void seri(TreeNode node, StringBuilder sb) {
        // exit
        if (node == null) {
            sb.append("null,");
            return;
        }
        sb.append("" + node.val + ",");
        seri(node.left, sb);
        seri(node.right, sb);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        Queue<String> q = new LinkedList<>();
        for (String s : data.split(",")) {
            q.offer(s);
        }
        return dese(q);
    }
    private TreeNode dese(Queue<String> q) {
        String str = q.poll();
        TreeNode root = "null".equals(str) ? null : new TreeNode(Integer.parseInt(str));
        if (root != null) {
            root.left = dese(q);
            root.right = dese(q);
        }
        return root;
    }
}

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

/*
1. BFS
2. DFS, pre-order
*/
```


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