野球拳刷刷刷: 297. Serialize and Deserialize Binary Tree

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

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.

Solution:

The idea is to use a queue to level order traverse the tree. If a node is not null, we append its value to the string. Otherwise we append a special character to indicate a null.

We also need to append a splitter after an append operation.

To decserializae, we also use a queue to store one level at a time.

If we meet a non-null string, we convert it to a node with value, and attach it to its parent. Then we add this node to the queue which will be used to level order at next round.

If we meet a null character, we simply ignore it.

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) {
        if (root == null) {
            return "";
        }
        StringBuilder sb = new StringBuilder();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0 ;i < size; i++) {
                TreeNode node = queue.poll();
                if (node == null) {
                    sb.append("#,");
                    continue;
                }
                sb.append(node.val + ",");
                queue.offer(node.left);
                queue.offer(node.right);
            }
        }
        sb.deleteCharAt(sb.length() - 1);
        return sb.toString();
    }

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

Monday, May 8, 2017

297. Serialize and Deserialize Binary Tree