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Problem solution · Java

Design a Todo List

Design a Todo List: a Java solution using hash-based lookup. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Hash-based lookup
Source
walkccc LeetCode Solutions
Length
50 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Hash-based lookup

For Design a Todo List, the implementation stores previously seen values or frequencies in a hash table for direct membership and lookup operations.

  1. Decide the key that represents the information needed later.
  2. Update its count or stored state while scanning the input.
  3. Use constant-time expected lookups to detect matches or assemble the result.

Code notes

  • 50 lines of Java from the credited upstream file 2590.java.
  • The implementation visibly relies on sequence storage, hash lookup, ordered lookup.
  • 3 loop blocks detected.

Complexity

Expected hash operations are constant time, but the surrounding scan and the number of stored keys determine total work and memory.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeDesign a Todo List · JavaJava
Use this to learn the idea, then write your own version.
class TodoList {  public int addTask(int userId, String taskDescription, int dueDate, List<String> tags) {    userIdToTaskIdToTasks.putIfAbsent(userId, new HashMap<>());    userIdToTaskIdToTasks.get(userId).put(++taskId, new Task(taskDescription, dueDate, tags));    taskIds.add(taskId);    return taskId;  }   public List<String> getAllTasks(int userId) {    List<String> res = new ArrayList<>();    for (Task task : getTasksSortedByDueDate(userId))      res.add(task.taskDescription);    return res;  }   public List<String> getTasksForTag(int userId, String tag) {    List<String> res = new ArrayList<>();    for (Task task : getTasksSortedByDueDate(userId))      if (task.tags.contains(tag))        res.add(task.taskDescription);    return res;  }   public void completeTask(int userId, int taskId) {    if (!taskIds.contains(taskId))      return;    if (!userIdToTaskIdToTasks.containsKey(userId))      return;    Map<Integer, Task> taskIdToTasks = userIdToTaskIdToTasks.get(userId);    if (!taskIdToTasks.containsKey(taskId))      return;    taskIdToTasks.remove(taskId);  }   private record Task(String taskDescription, int dueDate, List<String> tags) {}  private int taskId = 0;  private Set<Integer> taskIds = new HashSet<>();  // {userId: {taskId: tasks}}  private Map<Integer, Map<Integer, Task>> userIdToTaskIdToTasks = new HashMap<>();   private List<Task> getTasksSortedByDueDate(int userId) {    if (!userIdToTaskIdToTasks.containsKey(userId))      return new ArrayList<>();    TreeSet<Task> tasks = new TreeSet<>(Comparator.comparingInt(Task::dueDate));    for (Task task : userIdToTaskIdToTasks.get(userId).values())      tasks.add(task);    return new ArrayList<>(tasks);  }}

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