297. Serialize and Deserialize Binary Tree

Question

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. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.

Solution

编码过程

编码时采用BFS搜索，将二叉树按层级组成字符串。
当当前节点不等于null时，添加当前节点的值和一个符号。
然后将其左右子节点加入队列。
当当前节点为null时，添加一个非数字字符和一个符号。

解码过程

解码时同样采用BFS搜索，按层级恢复二叉树。
首先将字符串根据符号进行分割，变为字符串数组。
如果第一个字符表示为null，返回null。

否则将第一个数字组成根节点并加入队列。创建一个指针i记录位置。
当队列不为空，且指针i未越界时，根据指针i上的字符串创建当前节点的左子节点。
将指针右移，如果没有越界则创建当前节点的右子节点，然后将指针右移。

Code

/**
 * 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 data = new StringBuilder();
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        while(!q.isEmpty()){
            TreeNode curr = q.poll();
            if(curr == null) data.append("n,");
            else {
                data.append(curr.val + ",");
                q.add(curr.left);
                q.add(curr.right);
            }
        }
        return data.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        String[] values = data.split(",");
        if(values[0].equals("n")) return null;
        TreeNode root = new TreeNode(Integer.parseInt(values[0]));
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        int i = 1;     
        while(!q.isEmpty() && i < values.length){
            TreeNode curr = q.poll();
            curr.left = values[i].equals("n") ? null : new TreeNode(Integer.parseInt(values[i]));
            if(i++ < values.length) curr.right = values[i].equals("n") ? null : new TreeNode(Integer.parseInt(values[i]));
            if(curr.left != null) q.add(curr.left);
            if(curr.right != null) q.add(curr.right);
            i++;
        }
        return root;
    }
}

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