Array + String Problem Set for Senior Engineers

Here’s a curated list of challenging array and string problems that are ideal for Senior Software Engineer level candidates. These problems focus on problem-solving, optimization, edge-case handling, and deep understanding of data structures, often involving sliding windows, two pointers, hashing, dynamic programming, and custom sorting logic.

---

🔁 Two Pointers / Sliding Window

1. Longest Substring Without Repeating Characters
   Given a string, find the length of the longest substring without repeating characters.
   → Tags: Sliding Window, HashSet
   LeetCode 3

2. Minimum Window Substring
   Given two strings s and t, return the minimum window in s which will contain all the characters in t.
   → Tags: Sliding Window, HashMap
   LeetCode 76

3. Longest Subarray with Sum ≤ K (including negative numbers)
   Find the length of the longest subarray with sum less than or equal to K.
   → Tags: Prefix Sum, Monotonic Queue (advanced)

---

🧮 Prefix Sum / Difference Arrays

4. Subarray Sum Equals K
   Given an array of integers and an integer k, find the total number of continuous subarrays whose sum equals to k.
   → Tags: HashMap, Prefix Sum
   LeetCode 560

5. Range Sum Query with Updates (Mutable)
   Implement a data structure to perform range sum queries and point updates.
   → Tags: Segment Tree / Binary Indexed Tree
   LeetCode 307

---

🔄 In-Place Modifications / Reversals

6. Rotate Array
   Rotate the array to the right by k steps, in-place.
   → Tags: Array, Math
   LeetCode 189

7. Reverse Words in a String (No extra space)
   Reverse the words in a string in-place with O(1) extra space.
   → Tags: String, In-place
   LeetCode 151

---

🔍 Sorting & Searching

8. Median of Two Sorted Arrays
   Find the median of two sorted arrays of size m and n in O(log(m+n)).
   → Tags: Binary Search, Divide & Conquer
   LeetCode 4

9. Search in Rotated Sorted Array (with duplicates)
   Modified binary search where the array is rotated and may contain duplicates.
   LeetCode 81

---

🧩 Complex Logic / Combinatorics

10. Group Anagrams
    Group words that are anagrams of each other.
    → Tags: HashMap, Sorting
    LeetCode 49

11. Count Palindromic Substrings
    Count all palindromic substrings in a string.
    → Tags: DP, Expand Around Center
    LeetCode 647

12. Longest Valid Parentheses
    Given a string containing just '(' and ')', find the length of the longest valid (well-formed) parentheses substring.
    → Tags: Stack, DP
    LeetCode 32

---

🧠 Advanced / Optimization

13. Maximum Product Subarray
    Find the contiguous subarray within an array (containing at least one number) which has the largest product.
    → Tags: DP, Greedy
    LeetCode 152

14. Trapping Rain Water
    Given n non-negative integers representing elevation, compute how much water it can trap.
    → Tags: Two Pointers, Stack
    LeetCode 42

15. Word Ladder II
    Find all shortest transformation sequences from start to end word using a given dictionary.
    → Tags: BFS, Backtracking
    LeetCode 126

---

🧪 Bonus: Custom Implementation Challenges

16. Implement Substring Search (strStr / KMP Algorithm)
    Implement substring search function with O(n + m) time using KMP.
    LeetCode 28

17. Custom Sort String
    Given an order and a string s, sort s based on the custom order.
    LeetCode 791

18. Longest Consecutive Sequence (Unsorted Array)
    Find the length of the longest consecutive elements sequence.
    → Tags: HashSet, O(n)
    LeetCode 128

---

📊 Want These in a Table Format or Downloadable Doc?

Let me know if you want this list in:

• A Notion page / PDF / CSV

• With difficulty levels & estimated time per question

• With company tags (e.g. asked at Google, Meta, etc.)

Would you like that?