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297. Serialize and Deserialize Binary Tree

Description

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.

Clarification: The input/output format is the same as how LeetCode serializes a binary tree. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.

 

Example 1:

Input: root = [1,2,3,null,null,4,5]
Output: [1,2,3,null,null,4,5]

Example 2:

Input: root = []
Output: []

 

Constraints:

  • The number of nodes in the tree is in the range [0, 104].
  • -1000 <= Node.val <= 1000

Solutions

Solution 1: Level Order Traversal

We can use level order traversal to serialize the binary tree. Starting from the root node, we add the nodes of the binary tree to the queue in the order from top to bottom, from left to right. Then we dequeue the nodes in the queue one by one. If the node is not null, we add its value to the serialized string; otherwise, we add a special character #. Finally, we return the serialized string.

During deserialization, we split the serialized string by the delimiter to get a string array, and then add the elements in the string array to the queue in order. The elements in the queue are the nodes of the binary tree. We dequeue the elements from the queue one by one. If the element is not #, we convert it to an integer and use it as the value of the node, and then add the node to the queue; otherwise, we set it to null. Finally, we return the root node.

The time complexity is $O(n)$, and the space complexity is $O(n)$. Where $n$ is the number of nodes in the binary tree.

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# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None


class Codec:
    def serialize(self, root):
        """Encodes a tree to a single string.

        :type root: TreeNode
        :rtype: str
        """
        if root is None:
            return ""
        q = deque([root])
        ans = []
        while q:
            node = q.popleft()
            if node:
                ans.append(str(node.val))
                q.append(node.left)
                q.append(node.right)
            else:
                ans.append("#")
        return ",".join(ans)

    def deserialize(self, data):
        """Decodes your encoded data to tree.

        :type data: str
        :rtype: TreeNode
        """
        if not data:
            return None
        vals = data.split(",")
        root = TreeNode(int(vals[0]))
        q = deque([root])
        i = 1
        while q:
            node = q.popleft()
            if vals[i] != "#":
                node.left = TreeNode(int(vals[i]))
                q.append(node.left)
            i += 1
            if vals[i] != "#":
                node.right = TreeNode(int(vals[i]))
                q.append(node.right)
            i += 1
        return root


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

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/**
 * 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 null;
        }
        List<String> ans = new ArrayList<>();
        Deque<TreeNode> q = new LinkedList<>();
        q.offer(root);
        while (!q.isEmpty()) {
            TreeNode node = q.poll();
            if (node != null) {
                ans.add(node.val + "");
                q.offer(node.left);
                q.offer(node.right);
            } else {
                ans.add("#");
            }
        }
        return String.join(",", ans);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data == null) {
            return null;
        }
        String[] vals = data.split(",");
        int i = 0;
        TreeNode root = new TreeNode(Integer.valueOf(vals[i++]));
        Deque<TreeNode> q = new ArrayDeque<>();
        q.offer(root);
        while (!q.isEmpty()) {
            TreeNode node = q.poll();
            if (!"#".equals(vals[i])) {
                node.left = new TreeNode(Integer.valueOf(vals[i]));
                q.offer(node.left);
            }
            ++i;
            if (!"#".equals(vals[i])) {
                node.right = new TreeNode(Integer.valueOf(vals[i]));
                q.offer(node.right);
            }
            ++i;
        }
        return root;
    }
}

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