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Problem solution · Python

Design a Todo List

Design a Todo List: a Python solution using sorting and greedy selection. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Sorting and greedy selection
Source
walkccc LeetCode Solutions
Length
51 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

Sorting and greedy selection

For Design a Todo List, the implementation first exposes a useful order, then scans that order while making locally justified choices.

  1. Choose the key that reveals the greedy or grouping structure.
  2. Sort the relevant records by that key.
  3. Scan in order, maintaining the invariant that makes each local choice safe.

Code notes

  • 51 lines of Python from the credited upstream file 2590.py.
  • The implementation visibly relies on sequence storage, hash lookup, ordered lookup.
  • No explicit loop blocks detected.

Complexity

Sorting is typically the dominant term unless the subsequent scan uses a more expensive nested operation.

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 · PythonPython
Use this to learn the idea, then write your own version.
from dataclasses import dataclass  @dataclass(frozen=True)class Task:  taskDescription: str  dueDate: int  tags: list[str]  class TodoList:  def __init__(self):    self.taskId = 0    self.taskIds = set()    self.userIdToTaskIdToTasks: dict[int, dict[int, list[Task]]] = {}   def addTask(self, userId: int, taskDescription: str, dueDate: int,              tags: list[str]) -> int:    self.taskId += 1    taskIdToTasks = self.userIdToTaskIdToTasks.setdefault(userId, {})    taskIdToTasks[self.taskId] = Task(taskDescription, dueDate, tags)    self.taskIds.add(self.taskId)    return self.taskId   def getAllTasks(self, userId: int) -> list[str]:    return [task.taskDescription            for task in self._getTasksSortedByDueDate(userId)]   def getTasksForTag(self, userId: int, tag: str) -> list[str]:    return [task.taskDescription            for task in self._getTasksSortedByDueDate(userId)            if tag in task.tags]   def completeTask(self, userId: int, taskId: int) -> None:    if taskId not in self.taskIds:      return    if userId not in self.userIdToTaskIdToTasks:      return    taskIdToTasks = self.userIdToTaskIdToTasks[userId]    if taskId not in taskIdToTasks:      return    del taskIdToTasks[taskId]   def _getTasksSortedByDueDate(self, userId: int) -> list[Task]:    if userId not in self.userIdToTaskIdToTasks:      return []    taskIdToTasks = self.userIdToTaskIdToTasks[userId]    return sorted(        [task for task in taskIdToTasks.values()],        key=lambda x: x.dueDate)

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