297 Serialize and Deserialize Binary Tree

297. Serialize and Deserialize Binary Tree

1. 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.

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.

2. Implementation

/**
 * 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 res = new StringBuilder();
        convertToString(root, res);
        return res.toString();
    }

    public void convertToString(TreeNode node, StringBuilder res) {
        if (node == null) {
            res.append("#").append(",");
        }
        else {
            res.append(node.val).append(",");
            convertToString(node.left, res);
            convertToString(node.right, res);
        }
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        Queue<String> queue = new LinkedList<>();
        queue.addAll(Arrays.asList(data.split(",")));
        return buildTree(queue);
    }

    public TreeNode buildTree(Queue<String> queue) {
        String val = queue.remove();
        if (val.equals("#")) {
            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));

3. Time & Space Complexity

时间复杂度O(n), 空间复杂度O(n)