“Maintain a window or two moving indices while preserving an invariant.”

• The problem involves subarrays / substrings
• The data is sorted or can be sorted
• The question asks for longest / shortest / count
• You see words like:

30 MUST-DO INTERVIEW PROBLEMS

1. #
   Valid Palindrome – LC 125
2. #
   Two Sum II (Sorted Array) – LC 167
3. #
   Remove Duplicates from Sorted Array – LC 26
4. #
   Merge Sorted Array – LC 88
5. #
   Is Subsequence – LC 392
6. #
   Reverse String – LC 344
7. #
   Sort Array By Parity – LC 905
8. #
   Container With Most Water – LC 11
9. #
   Maximum Average Subarray I – LC 643
10. #
    Minimum Difference Between Highest and Lowest of K Scores – LC 1984

Pointer movement should feel

1. #
   Longest Substring Without Repeating Characters – LC 3
2. #
   Longest Repeating Character Replacement – LC 424
3. #
   Minimum Size Subarray Sum – LC 209
4. #
   Max Consecutive Ones III – LC 1004
5. #
   Fruits Into Baskets – LC 904
6. #
   Minimum Size Subarray in Infinite Array – LC 2401
7. #
   Subarrays with Product Less Than K – LC 713
8. #
   Permutation in String – LC 567
9. #
   Maximum Erasure Value – LC 1695
10. #
    Longest Substring with At Most K Distinct Characters – LC 340

• At most K
• Exactly K
• Frequency maps
• Shrinking logic
• Fixed size window
• Variable size window

Can you explain

1. #
   Minimum Window Substring – LC 76
2. #
   Sliding Window Maximum – LC 239
3. #
   Substring with Concatenation of All Words – LC 30
4. #
   Maximum Points You Can Obtain from Cards – LC 1423
5. #
   Minimum Number of K Consecutive Bit Flips – LC 995
6. #
   Longest Continuous Subarray with Absolute Diff ≤ Limit – LC 1438
7. #
   Constrained Subsequence Sum – LC 1425
8. #
   Find K Closest Elements – LC 658
9. #
   Count Number of Nice Subarrays – LC 1248
10. #
    Shortest Unsorted Continuous Subarray – LC 581

• Clear invariants
• Zero guesswork
• Calm boundary adjustment

The ONE Rule That Wins Interviews

“Why am I moving the left pointer now?”

Common Interview Mistakes

• Moving both pointers blindly
• Forgetting to shrink window
• Mixing prefix sum and sliding window without reason
• Not stating window invariant clearly

1️⃣ Pattern in One Line

"Maintain a window or two moving indices that preserve an invariant, avoiding redundant scanning."

2️⃣ Recognize It When...

• Problem involves subarrays/substrings
• Looking for longest/shortest/count with a condition
• Input is sorted OR can use two ends
• Trigger words: "at most K", "continuous", "window", "longest", "shortest"

3️⃣ Don't Confuse With...

• Prefix Sum (P3) → static precomputation vs dynamic window
• Monotonic Stack (P8) → when next greater/smaller is needed
• Sliding Window vs Two Pointers → SW has a window condition; TP works from both ends

4️⃣ Approach in 3 Steps

1. #
   Define the window invariant — what condition must the window always satisfy?
2. #
   Expand right pointer, shrink left pointer when invariant breaks
3. #
   Track answer at each valid window state

5️⃣ Invariant to Maintain

"At every step, the window [left, right] always satisfies the given condition."

6️⃣ Complexity to Justify

• Time: O(N) — each element enters and exits window at most once
• Space: O(1) or O(K) for frequency map
• Why: Two pointers never go backward

7️⃣ Edge Cases to Always Check

• Empty array/string
• Window larger than array
• All elements same
• K = 0 or K = N
• Negative numbers in sum problems

8️⃣ One Line to Say in Interview

"I'll use a sliding window — expand right to include elements, shrink left when the invariant breaks, giving O(N) time."