🚀 Introduction
Arrays (or lists in Python) and strings are among the most common data structures you’ll use. They form the backbone of many algorithms and interview questions.
In this post, we’ll explore:
✅ Python’s built-in list and string capabilities
✅ Essential algorithmic patterns: sliding window, two pointers, prefix sum
✅ Real-world problems and how to solve them
✅ Best practices and Pythonic shortcuts
📦 1️⃣ Python Lists: More Than Just Arrays
Python lists are dynamic arrays that can grow or shrink in size.
arr = [1, 2, 3] arr.append(4) # [1, 2, 3, 4] arr.pop() # [1, 2, 3] arr.insert(1, 10) # [1, 10, 2, 3] del arr[0] # [10, 2, 3] 🔹 List Slicing
nums = [1, 2, 3, 4, 5] print(nums[1:4]) # [2, 3, 4] print(nums[::-1]) # [5, 4, 3, 2, 1] – reversed ✅ Slicing is your best friend in array problems.
🔤 2️⃣ Strings in Python
Python strings are immutable and extremely versatile.
s = "hello" print(s.upper()) # "HELLO" print(s[1:4]) # "ell" print("h" in s) # True 🔹 String Tricks
# Reversing a string reversed_s = s[::-1] # Check for palindrome def is_palindrome(s): return s == s[::-1] 🚦 3️⃣ Pattern: Two Pointers
Use two pointers to scan a list or string from both ends or at different speeds.
🔹 Example: Is a list a palindrome?
def is_palindrome(lst): left, right = 0, len(lst) - 1 while left < right: if lst[left] != lst[right]: return False left += 1 right -= 1 return True 🔹 Example: Remove Duplicates from Sorted Array
def remove_duplicates(nums): if not nums: return 0 i = 0 for j in range(1, len(nums)): if nums[j] != nums[i]: i += 1 nums[i] = nums[j] return i + 1 ✅ Use when you need to compare or update positions in-place.
🔍 4️⃣ Pattern: Sliding Window
Efficiently track values over a moving range.
🔹 Example: Max sum of subarray of size k
def max_subarray_sum(nums, k): window_sum = sum(nums[:k]) max_sum = window_sum for i in range(k, len(nums)): window_sum += nums[i] - nums[i - k] max_sum = max(max_sum, window_sum) return max_sum ✅ Use when dealing with contiguous sequences like subarrays, substrings, etc.
➕ 5️⃣ Pattern: Prefix Sum
Prefix sum helps in range sum queries and cumulative processing.
🔹 Example: Range sum query
def prefix_sum(nums): prefix = [0] for num in nums: prefix.append(prefix[-1] + num) return prefix # Sum of elements from index i to j: prefix[j+1] - prefix[i] ✅ Reduces repeated computation in range-based problems.
🧪 6️⃣ Practice Problems
| Problem | Pattern | Tip |
|---|---|---|
| Two Sum | Hashing / Two Pointers | Use a set for complements |
| Maximum Subarray | Kadane’s Algorithm | Track max ending at current index |
| Longest Substring Without Repeating Characters | Sliding Window | Use a set or dict for seen chars |
| Palindrome Check | Two Pointers | Compare left and right |
| Product of Array Except Self | Prefix & Suffix Arrays | Avoid division |
✅ Best Practices
✔️ Know when to use in-place operations to save space
✔️ Use enumerate() to loop with index
✔️ Avoid += on strings in loops (use ''.join() instead)
✔️ Leverage collections.Counter, set, and defaultdict for advanced use cases
❌ Avoid nested loops unless necessary — try sliding window or hashing
🧠 Summary
✔️ Lists and strings are the foundation of most coding problems
✔️ Learn and practice patterns like two pointers, sliding window, and prefix sums
✔️ Python’s expressive syntax helps solve these problems cleanly and concisely
✔️ Build your DSA muscle memory with hands-on problems using these patterns
🔜 Coming Up Next:
👉 Part 3: Stacks and Queues – Theory, Python Implementations, and Interview Classics
We’ll explore:
- Real-world uses of stacks & queues
- How to implement with Python lists and deque
- Problems like parentheses validation, undo/redo, and level-order traversal
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