Most Important LeetCode 150 Interview Questions

📖 About

This repository contains comprehensive solutions to the 150 most important LeetCode questions frequently asked in technical interviews at top companies (FAANG and beyond). Each solution demonstrates optimal approaches, efficient algorithms, and best coding practices in Python. The problems are systematically organized by topic to facilitate structured interview preparation.

🎯 Achievement

Top 150 Leetcode Badge

🎯 Mission

Master the essential Data Structures and Algorithms patterns required to excel in coding interviews through systematic practice and understanding of core concepts.

📚 Problems Organized by Category

🟢 Arrays & Strings (24 Problems)

• 01_MergeSortedArrays.py - In-place merging with two pointers
• 02_RemoveElements.py - Two-pointer technique for element removal
• 03_RemoveDuplicatesFromSortedArray.py - Unique element extraction
• 04_RemoveDuplicateSortedArrayII.py - At most two duplicates allowed
• 05_MajorityElement.py - Boyer-Moore voting algorithm
• 06_RotatedArray.py - Array rotation with reversal technique
• 09_JumpGame.py - Greedy approach for reachability
• 10_JumpGame2.py - Minimum jumps with BFS-like approach
• 11_H_Index.py - Citation counting and sorting
• 12_Insert_Delete_GetRandom.py - O(1) operations with HashMap + ArrayList
• 15_Candy.py - Two-pass greedy algorithm
• 16_TrappingRainWater.py - Two-pointer approach for water trapping
• 17_RomanToInteger.py - Roman numeral conversion
• 18_IntegerToRoman.py - Greedy integer to Roman conversion
• 20_LongestCommonPrefix.py - Vertical scanning approach
• 23_FindTheIndexOfFirstOccurenceOfString.py - String matching
• 24_TextJustification.py - Complex text formatting algorithm

🟡 Two Pointers (5 Problems)

• 01_3Sum.py - Three-pointer approach for triplet finding
• 02_ValidPalindrome.py - Two-pointer palindrome validation
• 05_ContainerWithMostWater.py - Maximum area calculation with two pointers

🟡 Sliding Window (4 Problems)

• 01_MinimumSizeSubarraySum.py - Variable-size sliding window
• 02_longestSubstringWithoutRepeatingCharacters.py - HashMap-based sliding window
• 04_MinimumWindowSubstring.py - Complex sliding window with character frequency

🟡 Matrix (5 Problems)

• 01_SpiralMatrix.py - Layer-by-layer traversal
• 02_RotateMatrix.py - In-place rotation with transpose
• 04_SetZerosMatrix.py - O(1) space matrix modification
• 05_GameOfLife.py - In-place state transitions

🔵 Hashmap (7 Problems)

• 01_RansomNote.py - Character frequency counting
• 06_TwoSum.py - Classic two-sum with HashMap
• 07_HappyNumber.py - Cycle detection with HashSet

🟣 Intervals (2 Problems)

• 01_SummaryRanges.py - Consecutive number grouping
• 02_MergeIntervals.py - Interval merging with sorting

🟢 Stacks (5 Problems)

• 03_MinStack.py - O(1) min retrieval with auxiliary stack
• 04_EvaluateReversePolishNotation.py - Stack-based expression evaluation
• 05_BasicCalculator.py - Complex calculator implementation

🔵 Linked List (8 Problems)

• 03_MergeTwoSortedList.py - Two-pointer merge technique
• 08_RotateAList.py - Circular rotation with pointer manipulation

🟣 Binary Tree General (13 Problems)

• 01_MaximumDepthOfBinaryTree.py - Recursive depth calculation
• 07_PathSum.py - DFS for path finding
• 12_CountCompleteBinaryTree.py - Binary search on tree levels
• 13_BinaryTreeMaximumPathSum.py - Complex DFS with global maximum

🔴 Binary Search (7 Problems)

• 02_FindThePeakElement.py - Modified binary search
• 04_FindFirstAndLastPositionOfElementInSortedArray.py - Binary search variants
• 05_FindMinimumInRotatedSortedArray.py - Rotated array binary search
• 07_MedianOfTwoSortedArrays.py - Advanced binary search on two arrays

🟡 Heap (4 Problems)

• 01_KthLargestElementInArray.py - QuickSelect and heap approaches
• 02_IPO.py - Dual heap for capital maximization
• 03_FindKPairsWithSmallestSum.py - Min heap for K smallest pairs
• 04_FindMedianFromDataStream.py - Two heaps for running median

🔵 Bit Manipulation (6 Problems)

• 01_AddBinary.py - Binary addition with carry
• 03_NumberOf1Bits.py - Bit counting techniques
• 05_SingleNumber2.py - Bit manipulation for unique element
• 06_BitwiseAndOfANumber.py - Range bitwise AND

🟣 Math (5 Problems)

• MaxPointsOnLine.py - Slope calculation with HashMap
• PalindromeNumber.py - In-place palindrome check
• PlusOne.py - Array-based digit increment
• Pow(n,x).py - Binary exponentiation
• Sqrt(x).py - Binary search for square root

🔴 1D Dynamic Programming (5 Problems)

• ClimbingStairs.py - Classic Fibonacci DP
• HouseRobber.py - Non-adjacent element selection
• CoinChange.py - Unbounded knapsack variant
• LongestIncreasingSubsequence.py - O(n²) DP solution

🔴 Multidimensional Dynamic Programming (9 Problems)

• Triangle.py - Bottom-up DP on triangle
• MinimumPathSum.py - 2D grid path finding
• UniquePathII.py - Path counting with obstacles
• MaximalSquare.py - 2D DP for largest square
• InterleavingStrings.py - 2D string DP
• EditDistance.py - Classic edit distance DP
• BestTimeToBuyAndSellStockIII.py - 3D DP for stock trading
• BestTimeToBuyAndSellStockIV.py - K transactions DP

🟢 Backtracking (5 Problems)

• GenerateParenthesis.py - Recursive parenthesis generation
• CombinationsSum.py - Combination finding with backtracking
• Permutations.py - All permutations generation
• WordSearch.py - 2D board DFS with backtracking

🔵 Graph General (4 Problems)

• NumbersOfIsland.py - DFS/BFS for connected components
• SurroundedRegions.py - Border DFS approach
• EvaluateDivision.py - Graph with weighted edges

🟡 Graph BFS (3 Problems)

• SnakeAndLadder.py - BFS for shortest path
• MinimumGeneticMutation.py - BFS with transformations
• WordLadder.py - Bidirectional BFS

🟢 Trie (2 Problems)

• DesignAddandSearchWordsDataStructure.py - Trie with wildcard search

🔴 Divide and Conquer (4 Problems)

• ConvertSortedArrayToBinaryTree.py - BST construction from sorted array
• SortList.py - Merge sort on linked list
• ConstructQuadTree.py - Quad tree construction
• MergeKSortedArray.py - K-way merge with divide and conquer

🟣 Kadane's Algorithm (2 Problems)

• 01_MaximumSubarray.py - Classic Kadane's algorithm
• 02_MaximumSumCircularSubarray.py - Circular array variant

🛠️ Key Algorithms & Patterns Demonstrated

Two Pointers

• Fast and slow pointer technique
• Left and right pointer convergence
• Sliding window variations

Binary Search

• Classic binary search on sorted arrays
• Modified binary search for rotated arrays
• Binary search on answer space

Dynamic Programming

• Bottom-up tabulation
• Top-down memoization
• State space optimization
• 1D, 2D, and 3D DP problems

Graph Algorithms

• Depth-First Search (DFS)
• Breadth-First Search (BFS)
• Topological sorting
• Union-Find
• Shortest path algorithms

Tree Algorithms

• Pre-order, In-order, Post-order traversal
• Level-order traversal
• Binary Search Tree operations
• Tree construction and reconstruction

Greedy Algorithms

• Activity selection
• Jump game strategies
• Interval scheduling

Backtracking

• Combination generation
• Permutation generation
• Constraint satisfaction
• Pruning techniques

Advanced Data Structures

• Trie for string operations
• Heap/Priority Queue
• Monotonic Stack/Queue
• Disjoint Set Union (DSU)

📈 Difficulty Distribution

Difficulty	Count	Percentage
🟢 Easy	35	23%
🟡 Medium	85	57%
🔴 Hard	30	20%

🎯 Problem-Solving Patterns

Pattern 1: Two Pointers

When to use: Problems involving arrays/lists where you need to find pairs or triplets Examples: 3Sum, Container With Most Water

Pattern 2: Sliding Window

When to use: Problems involving subarrays/substrings with specific properties Examples: Minimum Window Substring, Longest Substring Without Repeating Characters

Pattern 3: Fast & Slow Pointers

When to use: Linked list cycle detection, finding middle element Examples: Linked list problems

Pattern 4: Merge Intervals

When to use: Overlapping intervals problems Examples: Merge Intervals

Pattern 5: Cyclic Sort

When to use: Problems involving arrays with numbers in a given range

Pattern 6: In-place Reversal of LinkedList

When to use: Reversing linked list segments

Pattern 7: Tree BFS

When to use: Level-order traversal, finding level averages Examples: Binary tree problems

Pattern 8: Tree DFS

When to use: Pre/In/Post-order traversal, path problems Examples: Path Sum, Maximum Path Sum

Pattern 9: Two Heaps

When to use: Finding median in a stream Examples: Find Median from Data Stream

Pattern 10: Subsets

When to use: Generating all combinations/permutations Examples: Permutations

Pattern 11: Modified Binary Search

When to use: Sorted/rotated arrays, search space reduction Examples: Search in Rotated Array

Pattern 12: Top K Elements

When to use: Finding K largest/smallest elements Examples: Kth Largest Element

Pattern 13: K-way Merge

When to use: Merging K sorted arrays/lists Examples: Merge K Sorted Lists

Pattern 14: Dynamic Programming

When to use: Optimization problems with overlapping subproblems Examples: Coin Change, Edit Distance

Pattern 15: Backtracking

When to use: Generating all possible solutions Examples: Generate Parentheses, Word Search

🚀 How to Use This Repository

1. Clone the Repository

git clone https://github.com/Ak-Rajak/Most_Imp_Interview_leetcode_Questions_150.git
cd Most_Imp_Interview_leetcode_Questions_150

2. Navigate to Specific Topic

cd Arrays_&_Strings

3. Run Any Solution

python 16_TrappingRainWater.py

4. Study the Approach

Each file contains:

• Problem link to LeetCode
• Detailed comments explaining the approach
• Time and space complexity analysis
• Multiple solution approaches (where applicable)

5. Example Study Flow

# From 16_TrappingRainWater.py
class Solution:
    def trap(self, height: List[int]) -> int:
        """
        Two-pointer approach:
        - Maintain leftMax and rightMax
        - Move pointer with smaller max
        - Calculate water at each position
        Time: O(n), Space: O(1)
        """
        if not height:
            return 0
        
        l, r = 0, len(height) - 1
        leftMax, rightMax = height[l], height[r]
        res = 0
        
        while l < r:
            if leftMax < rightMax:
                l += 1
                leftMax = max(leftMax, height[l])
                res += leftMax - height[l]
            else:
                r -= 1
                rightMax = max(rightMax, height[r])
                res += rightMax - height[r]
        
        return res

📊 Complexity Analysis Summary

Category	Average Time	Average Space
Arrays & Strings	O(n) - O(n²)	O(1) - O(n)
Binary Search	O(log n)	O(1)
Dynamic Programming	O(n²) - O(n³)	O(n) - O(n²)
Graphs	O(V + E)	O(V)
Trees	O(n)	O(h)
Heaps	O(n log k)	O(k)

🏆 Skills Gained

✅ Data Structure Mastery

• Arrays, Linked Lists, Trees, Graphs
• Stacks, Queues, Heaps
• Hash Tables, Tries
• Advanced structures (Segment Tree, Fenwick Tree)

✅ Algorithm Expertise

• Sorting and Searching algorithms
• Graph traversal (DFS, BFS)
• Dynamic Programming patterns
• Greedy algorithms
• Divide and Conquer

✅ Problem-Solving Skills

• Pattern recognition
• Time-space trade-off analysis
• Optimization techniques
• Edge case handling

✅ Interview Readiness

• Clear code communication
• Complexity analysis
• Multiple solution approaches
• Code optimization

🎓 Study Plan Recommendation

Week 1-2: Foundation (Easy & Medium)

• Arrays & Strings basics
• Two Pointers
• Hash Maps
• Basic Binary Search

Week 3-4: Intermediate

• Linked Lists
• Stacks & Queues
• Trees (Basic traversals)
• Sliding Window

Week 5-6: Advanced

• Dynamic Programming (1D)
• Binary Search (Advanced)
• Heaps
• Graphs (BFS/DFS)

Week 7-8: Expert

• Dynamic Programming (2D/3D)
• Backtracking
• Advanced Graph algorithms
• System Design considerations

📝 Interview Tips

Before the Interview

1. Review patterns: Focus on recognizing problem patterns
2. Practice articulation: Explain your thought process clearly
3. Time management: Practice solving problems within time limits
4. Mock interviews: Practice with peers or use platforms

During the Interview

1. Clarify requirements: Ask questions about edge cases
2. Discuss approach: Explain your strategy before coding
3. Start simple: Begin with brute force, then optimize
4. Test your code: Walk through test cases
5. Communicate: Think aloud throughout the process

Common Pitfalls to Avoid

• ❌ Jumping to code without planning
• ❌ Not considering edge cases
• ❌ Ignoring time/space complexity
• ❌ Not testing the solution
• ❌ Poor variable naming

🔗 Additional Resources

LeetCode

• LeetCode Top Interview 150
• LeetCode Discuss

Learning Platforms

• GeeksforGeeks
• InterviewBit
• AlgoExpert
• NeetCode

Books

• "Cracking the Coding Interview" by Gayle Laakmann McDowell
• "Elements of Programming Interviews in Python"
• "Introduction to Algorithms" by CLRS

YouTube Channels

• NeetCode
• Abdul Bari (Algorithms)
• Back To Back SWE
• Tech Dose

🤝 Contributing

Contributions are welcome! Here's how you can help:

1. Add alternative solutions: Different approaches to existing problems
2. Improve documentation: Better explanations and comments
3. Add test cases: More comprehensive testing
4. Fix bugs: Report and fix any issues
5. Optimize solutions: More efficient implementations

Contribution Guidelines

# Fork the repository
git fork https://github.com/Ak-Rajak/Most_Imp_Interview_leetcode_Questions_150.git

# Create a feature branch
git checkout -b feature/your-feature-name

# Commit your changes
git commit -m "Add: your feature description"

# Push to your branch
git push origin feature/your-feature-name

# Create a Pull Request

📄 License

This project is licensed under the MIT License - see the LICENSE file for details.

🌟 Star History

If you found this repository helpful, please consider giving it a star ⭐!

📞 Contact & Connect

• GitHub: Ak-Rajak
• LinkedIn: Your LinkedIn
• LeetCode: Your LeetCode Profile
• Email: your.email@example.com

🙏 Acknowledgments

• Thanks to LeetCode for providing an excellent platform for practice
• Inspired by countless developers in the community
• Special thanks to all contributors

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💡 "The only way to learn a new programming language is by writing programs in it." - Dennis Ritchie

Happy Coding! 🚀

Made with ❤️ and ☕ by Ak-Rajak

⭐ Star this repo if it helped you! ⭐