[LintCode] 7 Binary Tree Serialization 解题报告

Description
Design an algorithm and write code to serialize and deserialize a binary tree. Writing the tree to a file is called 'serialization' and reading back from the file to reconstruct the exact same binary tree is 'deserialization'.

Notice
There is no limit of how you deserialize or serialize a binary tree, LintCode will take your output of serialize as the input of deserialize, it won't check the result of serialize.


Example
An example of testdata: Binary tree {3,9,20,#,#,15,7}, denote the following structure:

  3
 / \
9  20
   /  \
 15   7
Our data serialization use bfs traversal. This is just for when you got wrong answer and want to debug the input.

You can use other method to do serializaiton and deserialization.



思路
serialize就level order一遍。遇到null就放#，否则就把node.val放上。注意前后有个大括号。
deserialize出来一共有values.length个点。那么我们for循环一遍。拿一个数字出来加到左儿子，拿一个数字出来加到右儿子。碰到#就不用管了。加完记得放进queue里。这个queue是用来level order的。每次拿一个出来，就可以往下加左儿子和右儿子。


Code

/**
 * 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) {
        // write your code here
        
        if (root == null) {
            return "{}";
        }
        
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        
        StringBuilder sb = new StringBuilder();
        sb.append("{");
        
        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);
        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) {
        // write your code here
        
        if (data == "{}") {
            return null;
        }
        
        Queue<TreeNode> queue = new LinkedList<>();
        String[] values = data.substring(1, data.length() - 1).split(",");
        
        TreeNode root = new TreeNode(Integer.parseInt(values[0]));
        queue.add(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;
    }
}