7.Binary Tree Serialization-二叉树的序列化和反序列化（中等题）

二叉树的序列化和反序列化

1. 题目

   设计一个算法，并编写代码来序列化和反序列化二叉树。将树写入一个文件被称为“序列化”，读取文件后重建同样的二叉树被称为“反序列化”。
   如何反序列化或序列化二叉树是没有限制的，你只需要确保可以将二叉树序列化为一个字符串，并且可以将字符串反序列化为原来的树结构。

2. 样例

   给出一个测试数据样例， 二叉树{3,9,20,#,#,15,7}，表示如下的树结构：
   
   我们的数据是进行BFS遍历得到的。当你测试结果wrong answer时，你可以作为输入调试你的代码。
   你可以采用其他的方法进行序列化和反序列化。

3. 题解

本题的序列化和反序列化都是二叉树的层序遍历问题，使用队列解决。先将头结点入列，将节点值序列化，再将其出列的同时检查其左右子节点是否为null，如不为null则入列，否则仅序列化即可，直至遍历完毕。

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
class Solution {
    /**
     * This method will be invoked first, you should design your own algorithm 
     * to serialize a binary tree which denote by a root node to a string which
     * can be easily deserialized by your own "deserialize" method later.
     */
    public String serialize(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        StringBuilder sb = new StringBuilder("");
        if (root == null)
        {
            return sb.toString();
        }
        queue.add(root);
        sb.append(String.valueOf(root.val) + ",");
        while (!queue.isEmpty())
        {
            TreeNode t = queue.poll();
            if (t.left != null)
            {
                queue.add(t.left);
            }
            sb.append(t.left == null ? "#," : String.valueOf(t.left.val) + ",");
            if (t.right != null)
            {
                queue.add(t.right);
            }
            sb.append(t.right == null ? "#" : String.valueOf(t.right.val));
            if (!queue.isEmpty())
            {
                sb.append(",");
            }
        }
        return sb.toString();
    }

    /**
     * This method will be invoked second, the argument data is what exactly
     * you serialized at method "serialize", that means the data is not given by
     * system, it's given by your own serialize method. So the format of data is
     * designed by yourself, and deserialize it here as you serialize it in 
     * "serialize" method.
     */
    public TreeNode deserialize(String data) {
        if (data.equals(""))
        {
            return null;
        }
        String[] nodeArr = data.split(",");
        TreeNode root = new TreeNode(Integer.valueOf(nodeArr[0]));
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        int index = 0;
        while (!queue.isEmpty())
        {
            TreeNode t = queue.poll();
            if (++index < nodeArr.length && !nodeArr[index].equals("#"))
            {
                t.left = new TreeNode(Integer.valueOf(nodeArr[index]));
                queue.add(t.left);
            }
            if (++index < nodeArr.length && !nodeArr[index].equals("#"))
            {
                t.right = new TreeNode(Integer.valueOf(nodeArr[index]));
                queue.add(t.right);
            }
        }

        return root;
    }
}

Last Update 2016.9.20