Serialize and Deserialize Binary Tree

// C — preorder with '#' for null, ',' as separator (conceptual)

// C++ — DFS Preorder
string serialize(TreeNode* root) {
    if (!root) return "#,";
    return to_string(root->val) + "," + serialize(root->left) + serialize(root->right);
}
TreeNode* deserialize(string data) {
    istringstream iss(data);
    function<TreeNode*()> build = [&]() -> TreeNode* {
        string tok;
        getline(iss, tok, ',');
        if (tok == "#") return nullptr;
        auto node = new TreeNode(stoi(tok));
        node->left = build();
        node->right = build();
        return node;
    };
    return build();
}

// Java — BFS level order
public String serialize(TreeNode root) {
    if (root == null) return "";
    StringBuilder sb = new StringBuilder();
    Queue<TreeNode> q = new LinkedList<>();
    q.add(root);
    while (!q.isEmpty()) {
        TreeNode node = q.poll();
        if (node == null) { sb.append("#,"); continue; }
        sb.append(node.val).append(',');
        q.add(node.left); q.add(node.right);
    }
    return sb.toString();
}
public TreeNode deserialize(String data) {
    if (data.isEmpty()) return null;
    String[] vals = data.split(",");
    TreeNode root = new TreeNode(Integer.parseInt(vals[0]));
    Queue<TreeNode> q = new LinkedList<>();
    q.add(root);
    int i = 1;
    while (!q.isEmpty() && i < vals.length) {
        TreeNode node = q.poll();
        if (!vals[i].equals("#")) { node.left = new TreeNode(Integer.parseInt(vals[i])); q.add(node.left); }
        i++;
        if (i < vals.length && !vals[i].equals("#")) { node.right = new TreeNode(Integer.parseInt(vals[i])); q.add(node.right); }
        i++;
    }
    return root;
}

// JavaScript — DFS preorder
function serialize(root) {
    if (!root) return '#,';
    return root.val + ',' + serialize(root.left) + serialize(root.right);
}
function deserialize(data) {
    const vals = data.split(',');
    let i = 0;
    function build() {
        if (vals[i] === '#') { i++; return null; }
        const node = new TreeNode(parseInt(vals[i++]));
        node.left = build();
        node.right = build();
        return node;
    }
    return build();
}

# Python — DFS preorder
def serialize(root):
    def dfs(node):
        if not node:
            return ['#']
        return [str(node.val)] + dfs(node.left) + dfs(node.right)
    return ','.join(dfs(root))

def deserialize(data):
    vals = iter(data.split(','))
    def build():
        val = next(vals)
        if val == '#':
            return None
        node = TreeNode(int(val))
        node.left = build()
        node.right = build()
        return node
    return build()