Coding Problems List

This is a curated list of coding problems I’ve worked on, with key insights and common mistakes I made.

Problems Completed

Two Sum

Tags: #array #hashtable #twopointer Status: ✅ Completed Key Insight: Hash Map for O(1) complement lookup. Demonstrates efficient Time-Space Trade-off.

Permutations

Tags: #backtracking #recursion #array Status: ✅ Completed Key Insight: Classic backtracking pattern with proper base case handling. Common Mistake: ⚠️ Return condition error - I was returning [] (empty list) instead of [[]] (list containing empty list) for the base case when nums is empty. The base case should return a list containing one permutation (the empty permutation), not an empty list of permutations.

Correct Base Case Logic:

def permute(nums):
    if not nums:
        return [[]]  # ✅ One empty permutation
        # NOT return []  # ❌ Zero permutations

Rationale: When nums = [], there is exactly one permutation of an empty list: the empty permutation []. So we return [[]] (a list containing that one permutation). This enables the recursive multiplication to work correctly.

Problems Queue

• 3Sum
• Combination Sum
• Subsets
• Word Search

Key Patterns & Insights

Backtracking Problems

• Base Case Pattern: Always consider what the “one valid result” should be for the simplest input
• Common Error: Confusing “no results” vs “one empty result” in base cases
• Template:

  def backtrack(path, remaining):
      if not remaining:  # base case
          return [path]  # one result containing current path
      # recursive case

Time-Space Trade-offs

• Hash Maps: O(1) lookup but O(N) space
• Two Pointers: O(1) space but requires sorted input
• Consider both approaches for complement/pair finding problems

Common Mistake Categories

1. Base Case Logic: Especially in recursive/backtracking problems
2. Off-by-one Errors: Array indexing and loop bounds
3. Edge Cases: Empty inputs, single elements, duplicates
4. Time Complexity: Not optimizing from brute force when possible