LeetCode Problem Solving — Core Concepts

The problem-solving process

Most people approach LeetCode wrong: they read the problem, stare at the screen, and either brute-force something or give up. Here is a systematic process that actually builds skill.

Step 1: Understand the problem (5 minutes)

Read the problem twice. Identify the input, output, and constraints. Work through the examples by hand. Ask yourself: “What would I do if I had to solve this with pen and paper?”

The constraints section is critical. If n ≤ 10⁵, you need O(n log n) or better. If n ≤ 20, even O(2ⁿ) is fine. The constraint tells you which approach is expected.

Step 2: Identify the pattern (5 minutes)

Most LeetCode problems map to a known pattern. Scan for signal words:

• “Subarray” or “substring” → sliding window
• “Sorted array” + “pair” → two pointers
• “All combinations” → backtracking
• “Minimum cost” or “number of ways” → dynamic programming
• “Connected components” or “grid” → BFS/DFS
• “K-th largest” → heap

Step 3: Plan before coding (5 minutes)

Write out your approach in comments or pseudocode. Define what data structures you need. Think about edge cases: empty input, single element, all duplicates, negative numbers.

Step 4: Implement (15 minutes)

Write clean Python. Use descriptive variable names — left, right, current_sum instead of i, j, s. This makes debugging easier and interviewers happier.

Step 5: Verify (5 minutes)

Trace through your code with the given examples. Then test edge cases mentally. Only then submit.

How to structure your study

Phase 1: Foundations (weeks 1-2)

Focus on Easy problems in these categories: arrays, strings, hash maps, two pointers. Solve 3-5 problems per category. Goal: get comfortable with Python syntax and basic patterns.

Key problems to start with: Two Sum, Valid Anagram, Best Time to Buy and Sell Stock, Valid Palindrome, Merge Two Sorted Lists.

Phase 2: Core patterns (weeks 3-6)

Move to Medium problems organized by pattern: sliding window, binary search, BFS/DFS, stack, linked lists. Solve 3-5 per pattern.

Goal: recognize patterns quickly and implement them without struggling with syntax.

Phase 3: Advanced patterns (weeks 7-10)

Tackle dynamic programming, backtracking, graphs, and tries. These are harder but follow templates. Start with the simplest DP problems (Climbing Stairs, Fibonacci) before attempting harder ones.

Phase 4: Integration (weeks 11-12)

Do timed practice. Set a 45-minute timer and solve 1-2 problems without looking anything up. This simulates interview conditions.

Python-specific advantages on LeetCode

Python has built-in tools that make LeetCode significantly easier:

Collections module:

• Counter for frequency counting
• defaultdict for grouping
• deque for BFS queues (O(1) popleft vs list’s O(n))

Built-in functions:

• sorted() with custom key functions
• zip() for parallel iteration
• enumerate() for index-value pairs
• any() and all() for condition checking

Comprehensions: List, dict, and set comprehensions express complex operations concisely. [x for x in nums if x > 0] is cleaner than a for loop with an if statement.

Negative indexing: nums[-1] gets the last element. nums[-2:] gets the last two. Saves you from nums[len(nums)-1].

When you are stuck

Being stuck is the most important part of learning. Here is a decision tree:

1. Stuck for 5 minutes: Re-read the problem. You probably missed a constraint.
2. Stuck for 15 minutes: Try brute force. Even an O(n³) solution helps you understand the problem structure.
3. Stuck for 25 minutes: Look at the problem’s tags (on LeetCode). They reveal the intended pattern.
4. Stuck for 30+ minutes: Read the editorial or top-voted solution. No shame — this is learning, not an exam.

After reading a solution, close it and implement it yourself from memory. If you cannot, you did not understand it yet.

Common misconception

“I need to solve 500+ problems to be ready.” Data from Blind and Teamblind surveys suggests that candidates who solve 100-150 problems with deep understanding perform as well as those who grind 400+ without internalizing patterns. The number matters less than the depth of your understanding.

Tracking progress

Keep a spreadsheet or notebook with columns: problem name, pattern used, whether you solved it alone, time taken, and a one-line note about the key insight. Reviewing this periodically reinforces patterns far more than solving new problems.

One thing to remember: LeetCode is not about solving problems — it is about recognizing patterns. Each problem you study should make the next similar problem easier. If it does not, you are memorizing, not learning.