AlgoExplained.dev

class Codec:
Defines the Codec class that provides two methods: serialize to convert a tree to a string, and deserialize to reconstruct the tree from a string.
    def serialize(self, root):
Serialize method takes the root of a binary tree and returns a comma-separated string representation using BFS level-order traversal.
        if not root:
Edge case: if the tree is empty (root is None), return an empty string. Deserialization will handle this by checking for an empty string input.
            return ""
Return empty string for an empty tree, signaling to deserialize that there is no tree to reconstruct.
        queue = deque([root])
Initialize a deque with the root node. This queue drives the BFS traversal, processing nodes level by level from top to bottom, left to right.
        result = []
List to collect the string representation of each node (including "null" for missing nodes) in BFS order.
        while queue:
BFS main loop: process every node in the queue, including null placeholders, until all levels are exhausted.
            node = queue.popleft()
Dequeue the next node. This could be a real TreeNode or None (a null placeholder enqueued from a previous step).
            if node:
If the node is not null, record its value and enqueue its children (even if they are null) to preserve structure.
                result.append(str(node.val))
Append the node's value as a string. This preserves the node's position in the level-order sequence.
                queue.append(node.left)
Enqueue the left child. If it's None, it will be output as "null" when dequeued, preserving the tree structure.
                queue.append(node.right)
Enqueue the right child. Both children are always enqueued to maintain the correct level-order positions.
            else:
If the node is null, record "null" to mark this position. No children are enqueued since a null node has no subtrees.
                result.append("null")
Append the "null" marker string, indicating no node exists at this position in the tree structure.
        return ",".join(result)
Join all collected values with commas to produce the final serialized string, e.g., "1,2,3,null,null,4,5".
    def deserialize(self, data):
Deserialize method takes a comma-separated string and reconstructs the original binary tree using BFS level-order logic.
        if not data:
Edge case: if the input string is empty, return None to represent an empty tree.
            return None
Return None for empty input, matching the serialization of an empty tree.
        nodes = data.split(",")
Split the serialized string by commas into an array of token strings. Each token is either a number or "null".
        root = TreeNode(int(nodes[0]))
Create the root TreeNode from the first token. The first value in a level-order serialization is always the root.
        queue = deque([root])
Initialize a queue with the root node. This queue holds nodes that need their left and right children assigned.
        i = 1
Index into the nodes array, starting at 1 (position 0 is the root). For each parent, nodes[i] is the left child and nodes[i+1] is the right child.
        while queue:
Process each parent node in the queue, assigning its left and right children from the serialized token list.
            node = queue.popleft()
Dequeue the next parent node that needs children assigned.
            if i < len(nodes) and nodes[i] != "null":
Check if there's a valid left child token. If within bounds and not "null", create a TreeNode for the left child.
                node.left = TreeNode(int(nodes[i]))
Create and assign the left child node from the token at index i.
                queue.append(node.left)
Enqueue the left child so its own children can be assigned later.
            i += 1
Advance the index past the left child token.
            if i < len(nodes) and nodes[i] != "null":
Check if there's a valid right child token. Same logic as the left child check.
                node.right = TreeNode(int(nodes[i]))
Create and assign the right child node from the token at the current index.
                queue.append(node.right)
Enqueue the right child so its own children can be assigned later.
            i += 1
Advance the index past the right child token. The next iteration will process the next parent.
        return root
Return the fully reconstructed tree root. The tree is now identical in structure and values to the original.

public class Codec {
Defines the Codec class providing serialize and deserialize methods for binary tree conversion to/from a string.
    public String serialize(TreeNode root) {
Serialize method that takes the root of a binary tree and returns a comma-separated BFS level-order string.
        if (root == null) return "";
Edge case: return empty string for a null root, signaling an empty tree during deserialization.
        Queue<TreeNode> q = new LinkedList<>();
Initialize a LinkedList-based queue for BFS traversal of the tree.
        q.offer(root);
Enqueue the root node to begin BFS traversal.
        StringBuilder sb = new StringBuilder();
StringBuilder for efficiently building the comma-separated output string.
        while (!q.isEmpty()) {
BFS loop: process every node in the queue until all levels are exhausted.
            TreeNode node = q.poll();
Dequeue the next node, which may be a real TreeNode or null.
            if (node != null) {
If the node exists, append its value and enqueue both children to preserve the tree structure.
                sb.append(node.val).append(",");
Append the node's value followed by a comma to the output string.
                q.offer(node.left);
Enqueue the left child (even if null) to preserve its position in the level-order sequence.
                q.offer(node.right);
Enqueue the right child (even if null) to preserve its position in the level-order sequence.
            } else {
If the node is null, append "null" as a placeholder marker.
                sb.append("null,");
Append "null," to mark this position as empty in the serialized string.
        return sb.toString();
Convert the StringBuilder to a String and return the complete serialized representation.
    public TreeNode deserialize(String data) {
Deserialize method that reconstructs the original binary tree from a comma-separated BFS string.
        if (data.isEmpty()) return null;
Edge case: return null for an empty input string, representing an empty tree.
        String[] nodes = data.split(",");
Split the serialized string by commas into an array of token strings.
        TreeNode root = new TreeNode(Integer.parseInt(nodes[0]));
Create the root TreeNode from the first token (always the root in BFS order).
        Queue<TreeNode> q = new LinkedList<>();
Queue of parent nodes that need their left and right children assigned from the token array.
        q.offer(root);
Enqueue the root as the first parent whose children need to be assigned.
        int i = 1;
Token index starting at 1 (root is at 0). For each parent, tokens i and i+1 are its left and right children.
        while (!q.isEmpty()) {
Process each parent in the queue, assigning children from the token array.
            TreeNode node = q.poll();
Dequeue the next parent node.
            if (i < nodes.length && !nodes[i].equals("null")) {
Check bounds and whether the left child token is a valid number (not "null").
                node.left = new TreeNode(Integer.parseInt(nodes[i]));
Create and assign the left child node from the token.
                q.offer(node.left);
Enqueue the left child so its children can be assigned in subsequent iterations.
            i++;
Advance the index past the left child token.
            if (i < nodes.length && !nodes[i].equals("null")) {
Check bounds and whether the right child token is a valid number.
                node.right = new TreeNode(Integer.parseInt(nodes[i]));
Create and assign the right child node from the token.
                q.offer(node.right);
Enqueue the right child for its own children to be assigned later.
            i++;
Advance the index past the right child token.
        return root;
Return the fully reconstructed tree root, identical in structure to the original tree.

class Codec {
Defines the Codec class with public serialize and deserialize methods for binary tree conversion.
public:
Public access specifier for the LeetCode-required entry point methods.
    string serialize(TreeNode* root) {
Serialize method: converts a binary tree into a comma-separated string using BFS level-order traversal.
        if (!root) return "";
Edge case: return empty string for a null root pointer.
        queue<TreeNode*> q;
Initialize a queue of TreeNode pointers for BFS traversal.
        q.push(root);
Push the root node onto the queue to begin BFS.
        string s;
String to accumulate the serialized output.
        while (!q.empty()) {
BFS main loop: process every node in the queue.
            TreeNode* node = q.front(); q.pop();
Retrieve and remove the front node from the queue.
            if (node) {
If the node is not null, append its value and enqueue both children.
                s += to_string(node->val) + ",";
Append the node's value followed by a comma to the output string.
                q.push(node->left);
Enqueue the left child (even if null) to preserve level-order positions.
                q.push(node->right);
Enqueue the right child (even if null) to preserve level-order positions.
            } else {
If the node is null, append "null" as a placeholder.
                s += "null,";
Append "null," to mark this position as empty in the serialized string.
        return s;
Return the complete serialized string.
    TreeNode* deserialize(string data) {
Deserialize method: reconstructs the binary tree from a comma-separated BFS string.
        if (data.empty()) return nullptr;
Edge case: return nullptr for an empty input string.
        vector<string> nodes;
Vector to hold the split token strings from the serialized data.
        stringstream ss(data);
Stringstream for tokenizing the comma-separated input string.
        string token;
Temporary string to hold each token as it's extracted.
        while (getline(ss, token, ',')) nodes.push_back(token);
Split the string by commas, pushing each token into the nodes vector. getline with ',' delimiter extracts one token at a time.
        TreeNode* root = new TreeNode(stoi(nodes[0]));
Create the root TreeNode from the first token. stoi converts the string to an integer.
        queue<TreeNode*> q;
Queue of parent nodes that need their children assigned from the token list.
        q.push(root);
Enqueue the root as the first parent needing children assigned.
        int i = 1;
Token index starting at 1 (root at 0). Each parent consumes two tokens for left and right children.
        while (!q.empty()) {
Process each parent, assigning left and right children from tokens at index i and i+1.
            TreeNode* node = q.front(); q.pop();
Dequeue the next parent node that needs children assigned.
            if (i < nodes.size() && nodes[i] != "null") {
Check bounds and whether the left child token is a valid value (not "null").
                node->left = new TreeNode(stoi(nodes[i]));
Create and assign the left child node.
                q.push(node->left);
Enqueue the left child for its own children to be assigned later.
            i++;
Advance index past the left child token.
            if (i < nodes.size() && nodes[i] != "null") {
Check bounds and whether the right child token is a valid value.
                node->right = new TreeNode(stoi(nodes[i]));
Create and assign the right child node.
                q.push(node->right);
Enqueue the right child for its own children to be assigned later.
            i++;
Advance index past the right child token.
        return root;
Return the fully reconstructed tree root, identical in structure to the original.