feat: solve Longest Repeating Character Replacement (#424)

README.md

@@ -6,7 +6,7 @@ A comprehensive collection of coding challenges from multiple platforms for lear
 
 | Platform                        | Focus Area                   | Problems Solved |
 | ------------------------------- | ---------------------------- | --------------- |
-| [LeetCode](#leetcode)           | Data Structures & Algorithms | 173             |
+| [LeetCode](#leetcode)           | Data Structures & Algorithms | 174             |
 | [GreatFrontEnd](#greatfrontend) | Frontend Engineering         | 15              |
 
 ## Platforms
@@ -175,7 +175,7 @@ Various places where I enjoy practicing solving coding challenges:
 </details>
 
 <details>
-<summary>Medium Problems (77 solved)</summary>
+<summary>Medium Problems (78 solved)</summary>
 
 ### Stack & Design
 
@@ -222,6 +222,7 @@ Various places where I enjoy practicing solving coding challenges:
 - [0187 - Repeated DNA Sequences](./leetcode/medium/0187-repeated-dna-sequences) ![Medium](https://img.shields.io/badge/Medium-orange)
 - [0340 - Longest Substring with At Most K Distinct Characters](./leetcode/medium/0340-longest-substring-with-at-most-k-distinct-characters) ![Medium](https://img.shields.io/badge/Medium-orange)
 - [0395 - Longest Substring with At Least K Repeating Characters](./leetcode/medium/0395-longest-substring-with-at-least-k-repeating-characters) ![Medium](https://img.shields.io/badge/Medium-orange)
+- [0424 - Longest Repeating Character Replacement](./leetcode/medium/0424-longest-repeating-character-replacement) ![Medium](https://img.shields.io/badge/Medium-orange)
 - [0438 - Find All Anagrams in a String](./leetcode/medium/0438-find-all-anagrams-in-a-string) ![Medium](https://img.shields.io/badge/Medium-orange)
 - [0616 - Add Bold Tag in String](./leetcode/medium/0616-add-bold-tag-in-string) ![Medium](https://img.shields.io/badge/Medium-orange)
 - [0692 - Top K Frequent Words](./leetcode/medium/0692-top-k-frequent-words) ![Medium](https://img.shields.io/badge/Medium-orange)
@@ -334,6 +335,7 @@ Various places where I enjoy practicing solving coding challenges:
 - [0159 - Longest Substring with At Most Two Distinct Characters](./leetcode/medium/0159-longest-substring-with-at-most-two-distinct-characters) - Medium
 - [0209 - Minimum Size Subarray Sum](./leetcode/medium/0209-minimum-size-subarray-sum) - Medium
 - [0340 - Longest Substring with At Most K Distinct Characters](./leetcode/medium/0340-longest-substring-with-at-most-k-distinct-characters) - Medium
+- [0424 - Longest Repeating Character Replacement](./leetcode/medium/0424-longest-repeating-character-replacement) - Medium
 - [1084 - Find K-Length Substrings With No Repeated Characters](./leetcode/medium/1084-find-k-length-substrings-with-no-repeated-characters) - Medium
 - [More in docs →](./docs/techniques/SLIDING_WINDOW.md)
 

leetcode/medium/0424-longest-repeating-character-replacement/POST_MORTEM.md

@@ -0,0 +1,193 @@
+# Post-Mortem Log
+
+## Problem
+
+**[Longest Repeating Character Replacement](https://leetcode.com/problems/longest-repeating-character-replacement/)** (LeetCode #424)
+
+Given a string `s` of uppercase English letters and an integer `k`, find the length of the longest substring containing the same letter after performing at most `k` character replacements.
+
+- **Problem Name:** Longest Repeating Character Replacement
+- **Problem Link:** <https://leetcode.com/problems/longest-repeating-character-replacement/>
+- **Date:** 2026-02-06
+
+---
+
+## Time Tracking
+
+- **Time to design the algorithm:** ~5 minutes (pattern recognition was straightforward)
+- **Time to code:** ~15 minutes (5 TDD cycles, incremental implementation)
+- **Time debugging/fixing:** ~5 minutes (minimal - TDD caught issues early)
+
+---
+
+## Solution Exploration
+
+### What approaches did I consider?
+
+1. **Brute Force** - Check all possible substrings
+   - For each starting position, try all ending positions
+   - For each substring, count characters and check if replacements ≤ k
+   - Time: O(n²) or O(n³) depending on implementation
+   - Space: O(1) or O(26)
+
+2. **Optimized Sliding Window** (Final Solution)
+   - Use two pointers to maintain a window
+   - Track character frequencies in the current window
+   - A window is valid when: `window_length - max_frequency ≤ k`
+   - Expand right pointer, shrink left when invalid
+   - Time: O(n), Space: O(1)
+
+### Final Solution Analysis
+
+- **Time Complexity:** O(n) - single pass through the string with two pointers
+- **Space Complexity:** O(1) - frequency map has at most 26 uppercase letters (constant)
+- **Is it optimal?** Yes - we must examine each character at least once, so O(n) is optimal
+
+### Why this approach worked (or didn't)
+
+The sliding window approach works because:
+
+- We only need to track the maximum frequency character in each window
+- The key insight is that `window_length - max_frequency` tells us how many replacements we need
+- As we slide the window, we maintain this invariant: needed replacements ≤ k
+- We don't need to recalculate max frequency from scratch when shrinking - we just use the running max
+
+**Trade-offs:**
+
+- Sliding window requires careful boundary management
+- But it's much more efficient than brute force
+- The "trick" is realizing we don't need to recalculate maxFreq when shrinking (using the historical max is sufficient)
+
+---
+
+## Pattern Recognition
+
+### What algorithmic pattern does this problem use?
+
+**Variable-Size Sliding Window** with constraint tracking
+
+### Key Insight
+
+The "aha moment" is realizing that a window is valid if and only if:
+
+```text
+window_length - max_frequency_in_window ≤ k
+```
+
+This formula tells us: "If I keep the most common character and replace all others, do I stay within k replacements?"
+
+We don't need to track which character to keep or replace - just count frequencies and apply this formula.
+
+### Related Problems
+
+1. **LeetCode 3: Longest Substring Without Repeating Characters**
+   - Similar sliding window pattern, different constraint
+
+2. **LeetCode 1004: Max Consecutive Ones III**
+   - Nearly identical problem structure (can flip at most k zeros)
+
+3. **LeetCode 340: Longest Substring with At Most K Distinct Characters**
+   - Variable sliding window with k constraint
+
+4. **LeetCode 209: Minimum Size Subarray Sum**
+   - Variable sliding window finding minimum instead of maximum
+
+---
+
+## Edge Cases & Verification
+
+### What clarifying questions did I ask?
+
+- What if k = 0? (No replacements allowed)
+- What if k is very large? (Can replace entire string)
+- What if the string is already all one character?
+- What about single-character strings?
+
+### Edge cases handled
+
+- ✅ All characters different, k=0 → returns 1
+- ✅ All characters the same → returns string length
+- ✅ Single character string → returns 1
+- ✅ k larger than needed → returns full string length
+- ✅ Both example cases from problem statement
+
+### Edge cases missed
+
+- None identified - test coverage appears comprehensive
+
+---
+
+## Mistakes & Bugs
+
+### Mistakes I keep making
+
+- Initial hardcoded return in Cycle 1 was intentional (TDD principle)
+- No significant bugs encountered during implementation
+- The TDD approach prevented common sliding window mistakes:
+  - Off-by-one errors in window size calculation
+  - Forgetting to update frequency map when shrinking window
+  - Not handling edge cases incrementally
+
+### Bugs to add to the Bug List
+
+- N/A - clean implementation from the start due to TDD
+
+---
+
+## Retrospective
+
+### Key Takeaways & Lessons Learned
+
+1. **TDD forces simplicity first** - Starting with a hardcoded value felt silly but led to a natural progression
+2. **The window validity formula is the key** - Once you understand `len - maxFreq ≤ k`, implementation is straightforward
+3. **Edge case testing prevented bugs** - Adding tests for k=0, single char, all same chars caught potential issues early
+4. **Pattern recognition accelerates solving** - Recognizing this as a sliding window problem immediately suggested the approach
+5. **TDD cycles build confidence** - Each passing test confirmed the algorithm was on the right track
+
+### Add to Cheat Sheet
+
+**Variable Sliding Window Template:**
+
+```typescript
+function slidingWindow(s: string, k: number): number {
+  const freq: Record<string, number> = {};
+  let maxFreq = 0;
+  let maxResult = 0;
+  let left = 0;
+
+  for (let right = 0; right < s.length; right++) {
+    // Expand: add right element
+    freq[s[right]] = (freq[s[right]] || 0) + 1;
+    maxFreq = Math.max(maxFreq, freq[s[right]]);
+
+    // Shrink: while window is invalid
+    while (WINDOW_INVALID_CONDITION) {
+      freq[s[left]]--;
+      left++;
+    }
+
+    // Update result
+    maxResult = Math.max(maxResult, right - left + 1);
+  }
+
+  return maxResult;
+}
+```
+
+**Key Formula for this problem:**
+
+- Window is valid when: `windowLength - maxFreq ≤ k`
+- Window size: `right - left + 1`
+
+---
+
+## Rubric Self-Rating (1–5)
+
+| Category            | Rating | Notes                                                                  |
+| ------------------- | ------ | ---------------------------------------------------------------------- |
+| **Problem solving** | 5      | Correctly identified pattern, understood key insight, optimal solution |
+| **Coding**          | 5      | Clean implementation, good variable names, proper TypeScript types     |
+| **Verification**    | 5      | Comprehensive test coverage, all edge cases handled                    |
+| **Communication**   | 5      | Clear test names, well-commented code, followed TDD discipline         |
+
+---

leetcode/medium/0424-longest-repeating-character-replacement/README.md

@@ -0,0 +1,28 @@
+# [Longest Repeating Character Replacement](https://leetcode.com/problems/longest-repeating-character-replacement) ![Medium](https://img.shields.io/badge/Medium-orange)
+
+You are given a string `s` and an integer `k`. You can choose any character of the string and change it to any other uppercase English character. You can perform this operation at most `k` times.
+
+Return _the length of the longest substring containing the same letter you can get after performing the above operations_.
+
+**Example 1:**
+
+```bash
+Input: s = "ABAB", k = 2
+Output: 4
+Explanation: Replace the two 'A's with two 'B's or vice versa.
+```
+
+**Example 2:**
+
+```bash
+Input: s = "AABABBA", k = 1
+Output: 4
+Explanation: Replace the one 'A' in the middle with 'B' and form "AABBBBA".
+The substring "BBBB" has the longest repeating letters, which is 4.
+```
+
+**Constraints:**
+
+- `1 <= s.length <= 10^5`
+- `s` consists of only uppercase English letters.
+- `0 <= k <= s.length`

leetcode/medium/0424-longest-repeating-character-replacement/longest-repeating-character-replacement.test.ts

@@ -0,0 +1,28 @@
+import { describe, it, expect } from 'vitest';
+import { characterReplacement } from './longest-repeating-character-replacement';
+
+describe('Longest Repeating Character Replacement', () => {
+  it('should return 4 for "ABAB" with k=2', () => {
+    expect(characterReplacement('ABAB', 2)).toBe(4);
+  });
+
+  it('should return 4 for "AABABBA" with k=1', () => {
+    expect(characterReplacement('AABABBA', 1)).toBe(4);
+  });
+
+  it('should return 1 when k=0 and all characters are different', () => {
+    expect(characterReplacement('ABCD', 0)).toBe(1);
+  });
+
+  it('should return full length when all characters are the same', () => {
+    expect(characterReplacement('AAAA', 0)).toBe(4);
+  });
+
+  it('should handle single character string', () => {
+    expect(characterReplacement('A', 1)).toBe(1);
+  });
+
+  it('should return full string length when k is large enough to replace all', () => {
+    expect(characterReplacement('ABCDE', 10)).toBe(5);
+  });
+});

leetcode/medium/0424-longest-repeating-character-replacement/longest-repeating-character-replacement.ts

@@ -0,0 +1,31 @@
+/**
+ * Finds the length of the longest substring with the same letter after at most k replacements
+ * @param s - Input string consisting of uppercase English letters
+ * @param k - Maximum number of character replacements allowed
+ * @returns Length of the longest valid substring
+ */
+export function characterReplacement(s: string, k: number): number {
+  const charCount: Record<string, number> = {};
+  let maxFreq = 0;
+  let maxLength = 0;
+  let left = 0;
+
+  for (let right = 0; right < s.length; right++) {
+    // Add current character to window
+    charCount[s[right]] = (charCount[s[right]] || 0) + 1;
+
+    // Update max frequency in current window
+    maxFreq = Math.max(maxFreq, charCount[s[right]]);
+
+    // If window is invalid (need more than k replacements), shrink from left
+    while (right - left + 1 - maxFreq > k) {
+      charCount[s[left]]--;
+      left++;
+    }
+
+    // Update max length
+    maxLength = Math.max(maxLength, right - left + 1);
+  }
+
+  return maxLength;
+}