297. 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.

Example:

You may serialize the following tree:

1

/
2 3
/
4 5

as “[1,2,3,null,null,4,5]”
Clarification: The above 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.

Note: Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.

Problem URL

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Solution

将一棵二叉树序列化，也就是转成String，然后还能转换回来。

Using pre-order traversal to add node val and spliter into a StringBuilder. If node == null, add NN. Return this string.

For deserialize, Using a queue to contain the String split by spliter to list. Then add them recursively, same as pre-order traversal.

Code

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {
    private static final String NN = "null";
    private static final String spliter = ",";
    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        buildString(root, sb);
        return sb.toString();
    }
    
    private void buildString(TreeNode node, StringBuilder sb){
        if (node == null){
            sb.append(NN).append(spliter);
            return;
        }
        sb.append(node.val).append(spliter);
        buildString(node.left, sb);
        buildString(node.right, sb);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        Queue<String> queue = new LinkedList<>();
        queue.addAll(Arrays.asList(data.split(spliter)));
        return buildTree(queue);
    }
    
    private TreeNode buildTree(Queue<String> queue){
        String val = queue.remove();
        if (val.equals(NN)){
            return null;
        }
        TreeNode node = new TreeNode(Integer.valueOf(val));
        node.left = buildTree(queue);
        node.right = buildTree(queue);
        return node;
    }
}

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

Time Complexity: O(n)
Space Complexity: O(n)

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