Serialize and Deserialize N-ary Tree

Tree, Design
Hard
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 following3-arytree
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. 1.
    N is in the range of [1, 1000]
  2. 2.
    Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.

Solution

DFS - Recursion (Preferred)

和 Serialize and Deserialize Binary Tree 很相似,同样可以用pre-order结合DFS来实现。
区别在于,在root value之后要append一个children count,这样才可以方便deserialize。
This is a template that can be applied to serialize and deserialize both binary and n-ary trees.
The only difference is that to serialize n-ary tree, we need to append the number of children of a node.
Serialize output with example as input:
1,3,3,2,5,0,6,0,2,0,4,0,
--
/*
// 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) {
StringBuilder sb = new StringBuilder();
serial(sb, root);
// System.out.println(sb.toString());
return sb.toString();
}
private void serial(StringBuilder sb, Node root) {
if (root == null) {
sb.append("#");
sb.append(",");
} else {
sb.append(root.val);
sb.append(",");
if (root.children != null) {
sb.append(root.children.size());
sb.append(",");
for (Node child: root.children) {
serial(sb, child);
}
}
}
}
// Decodes your encoded data to tree.
public Node deserialize(String data) {
Queue<String> queue = new LinkedList<String>(Arrays.asList(data.split(",")));
return buildTree(queue);
}
private Node buildTree(Queue<String> queue) {
String val = queue.poll();
if (val.equals("#")) {
return null;
}
Node root = new Node(Integer.parseInt(val));
int childrenCount = Integer.parseInt(queue.poll());
root.children = new ArrayList<Node>();
for (int i = 0; i < childrenCount; 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));

Pre-order Depth-first search with Stack

example as input
serialize结果
1[3[5[]6[]]2[]4[]]
--
class Codec {
// Encodes a tree to a single string.
public String serialize(Node root) {
if (root == null) {
return "";
}
StringBuilder sb = new StringBuilder();
sb.append(root.val);
sb.append("[");
for (Node child : root.children) {
sb.append(serialize(child));
}
sb.append("]");
return sb.toString();
}
// Decodes your encoded data to tree.
public Node deserialize(String data) {
Node root = null;
Stack<Node> stack = new Stack<>();
int i = 0;
while (i < data.length()) {
int start = i;
// Move pointer forward until we don't find a digit...
while (i < data.length() && Character.isDigit(data.charAt(i))) {
i++;
}
// If we haven't found a digit then we must have found the end of a child list...
if (start == i) {
Node child = stack.pop();
if (stack.isEmpty()) {
root = child;
break;
} else {
// Remove the child from the stack and assign it to the previous node on the stack
stack.peek().children.add(child);
}
} else {
Node n = new Node();
n.val = Integer.parseInt(data.substring(start, i));
n.children = new ArrayList<>();
stack.push(n);
}
i++;
}
return root;
}
}

Reference