Leetcode #2030: Smallest K-Length Subsequence With Occurrences of a Letter

In this guide, we solve Leetcode #2030 Smallest K-Length Subsequence With Occurrences of a Letter in Python and focus on the core idea that makes the solution efficient.

You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.

Problem Statement

You are given a string s, an integer k, a letter letter, and an integer repetition. Return the lexicographically smallest subsequence of s of length k that has the letter letter appear at least repetition times.

Quick Facts

• Difficulty: Hard
• Premium: No
• Tags: Stack, Greedy, String, Monotonic Stack

Intuition

A locally optimal choice leads to a globally optimal result for this structure.

That means we can commit to decisions as we scan without backtracking.

Approach

Sort or preprocess if needed, then repeatedly take the best available local choice.

Maintain the minimal state necessary to validate the greedy decision.

Steps:

• Sort or preprocess as needed.
• Iterate and pick the best local option.
• Track the current solution.

Example

Python Solution

Complexity

The time complexity is O(n log n). The space complexity is O(1) to O(n).

Edge Cases and Pitfalls

Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.

Summary

This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.