[leetcode]428. Serialize and Deserialize N-ary Tree序列化与反序列化N叉树

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 an N-ary tree. An N-ary tree is a rooted tree in which each node has no more than N children. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that an N-ary 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 3-ary tree

 

as [1 [3[5 6] 2 4]]. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.

Note:

1. N is in the range of [1, 1000]
2. Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.

思路

1.  preorder recursive traversal

2. add number of children after root val, in order to know when to terminate 

1 3 3 2 5 0 6 0 2 0 4 0 

代码

/*
// Definition for a Node.
class Node {
    public int val;
    public List<Node> children;

    public Node() {}

    public Node(int _val,List<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Codec {
 // Encodes a tree to a single string.
    public String serialize(Node root) {
        List<String> list = new LinkedList<>();
        buildString(root, list);
        return String.join(",", list);
    }

    private void buildString(Node root, List<String> list) {
        if (root == null) return;

        list.add(String.valueOf(root.val));
        list.add(String.valueOf(root.children.size()));
        for (Node child : root.children) {
            buildString(child, list);
        }

    }

    // Decodes your encoded data to tree.
    public Node deserialize(String data) {
        if (data.length() == 0) return null;
        String[] strArr = data.split(",");
        Queue<String> queue = new LinkedList<>();
        Collections.addAll(queue, strArr);
        return buildTree(queue);
    }

    private Node buildTree(Queue<String> queue) {
        // match the given constructor form
        Node root = new Node();
        root.val = Integer.parseInt(queue.poll());
        int size = Integer.parseInt(queue.poll());
        root.children = new ArrayList<>(size);
        for (int i = 0; i < size; i++) {
            root.children.add(buildTree(queue));
        }
        return root;
    }
}


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