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.

For example, you may serialize the following tree

tree

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.

Solution

BFS

Level order traversal，形式跟leetcode oj的完全相同，即null尽量省略。

/**
 * 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) {
        if (root == null) {
            return "";
        }
        StringBuilder sb = new StringBuilder();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        
        while (!queue.isEmpty()) {
            int size = queue.size();
            Queue<TreeNode> nextLevel = new LinkedList<>();
            boolean nextLevelExists = false; // whether there's non-null node in next level
            
            while (size-- > 0) {
                TreeNode node = queue.poll();
                sb.append(sb.length() > 0 ? "," : "");
                    
                if (node == null) {
                    sb.append("null");
                    continue;
                }

                sb.append(node.val);
                nextLevel.offer(node.left);
                nextLevel.offer(node.right);
                nextLevelExists |= node.left != null || node.right != null;
            }
                        
            if (nextLevelExists) {  // in order to omit extra nulls in the end
                queue = nextLevel;
            }
        }
        
        return sb.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data == null || data.isEmpty()) {
            return null;
        }
        
        String[] tokens = data.split(",");
        TreeNode root = new TreeNode(Integer.parseInt(tokens[0]));
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        
        for (int i = 1; i < tokens.length; ++i) {
            TreeNode node = queue.poll();

            if (!"null".equals(tokens[i])) {
                TreeNode left = new TreeNode(Integer.parseInt(tokens[i]));
                node.left = left;
                queue.offer(left);
            }

            if (!"null".equals(tokens[++i])) {
                TreeNode right = new TreeNode(Integer.parseInt(tokens[i]));
                node.right = right;
                queue.offer(right);
            }
        }
        
        return root;
    }
}

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

DFS

利用preorder traversal。deserialize的时候将tokens存入queue是个不错的想法，这样每次从queue取出一个string来就行，就不需要保存当前处理到的index了。

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

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        Queue<String> queue = new LinkedList<>();
        queue.addAll(Arrays.asList(data.split(",")));
        return dfsDeserialize(queue);
    }
    
    private TreeNode dfsDeserialize(Queue<String> queue) {
        String s = queue.poll();
        if ("null".equals(s)) {
            return null;
        }
        
        TreeNode node = new TreeNode(Integer.parseInt(s));
        node.left = dfsDeserialize(queue);
        node.right = dfsDeserialize(queue);
        
        return node;
    }
}

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