I Solved 100+ LeetCode Problems — These Are the Patterns That Actually Matter

Duong Tran Quang — Sun, 14 Jun 2026 19:36:39 +0000

I Solved 100+ LeetCode Problems — These Are the Patterns That Actually Matter

When I first started solving LeetCode problems, I made the same mistake many beginners make:

I tried to memorize solutions.

That approach worked for a few problems, but completely broke down once I encountered new variations.

After solving more than 100 problems, I realized that most interview questions are not testing specific solutions. They are testing whether you recognize the underlying pattern.

Instead of asking:

How do I solve this problem?

I started asking:

Which pattern is this problem testing?

That small shift changed everything.

1. Arrays & Hashing

Common problems:

Two Sum
Contains Duplicate
Valid Anagram
Group Anagrams
Top K Frequent Elements

Core idea:

Use hash maps and hash sets to trade memory for speed.

Typical complexity target:

Time: O(n)
Space: O(n)

2. Two Pointers

Common problems:

Valid Palindrome
Two Sum II
Container With Most Water
3Sum

Core idea:

Move two indices through a sorted structure or from opposite ends of a sequence.

Typical complexity target:

Time: O(n)
Space: O(1)

3. Sliding Window

Common problems:

Best Time to Buy and Sell Stock
Longest Substring Without Repeating Characters
Minimum Window Substring
Permutation in String

Core idea:

Expand and shrink a window while maintaining constraints.

Typical complexity target:

Time: O(n)

4. Stack

Common problems:

Valid Parentheses
Min Stack
Daily Temperatures
Largest Rectangle in Histogram

Core idea:

Maintain information about previously seen elements.

Special pattern:

Monotonic Stack.

5. Binary Search

Common problems:

Binary Search
Search a 2D Matrix
Search in Rotated Sorted Array
Koko Eating Bananas

Core idea:

Search on answer space, not just on arrays.

This was one of the most important interview patterns for me.

6. Trees

Common problems:

Maximum Depth of Binary Tree
Same Tree
Balanced Binary Tree
Validate Binary Search Tree

Core idea:

Most tree questions become straightforward once you become comfortable with recursive DFS.

7. Heap / Priority Queue

Common problems:

Kth Largest Element in an Array
Last Stone Weight
K Closest Points to Origin
Find Median from Data Stream

Core idea:

Efficiently maintain the smallest or largest elements.

8. Backtracking

Common problems:

Subsets
Permutations
Combination Sum
N-Queens

Core idea:

Build a decision tree and explore all possibilities.

9. Graphs

Common problems:

Number of Islands
Clone Graph
Course Schedule
Word Ladder

Core idea:

Most graph problems reduce to DFS, BFS, or Topological Sort.

10. Dynamic Programming

Common problems:

Climbing Stairs
House Robber
Coin Change
Longest Increasing Subsequence

Core idea:

Store solutions to smaller subproblems and reuse them.

Final Thoughts

The biggest improvement in my interview preparation came from organizing problems by pattern rather than by difficulty.

Once you recognize the pattern, the implementation becomes much easier.

If you're currently learning LeetCode, focus less on memorizing solutions and more on identifying the recurring techniques behind them.

What pattern took you the longest to understand?

I recently organized detailed notes, explanations, and visual walkthroughs for these patterns on my personal site:

https://dtducas.com/blog

I'm continuously adding new problems and explanations as I work through them.

DEV Community: Duong Tran Quang

I Solved 100+ LeetCode Problems — These Are the Patterns That Actually Matter

I Solved 100+ LeetCode Problems — These Are the Patterns That Actually Matter

1. Arrays & Hashing

2. Two Pointers

3. Sliding Window

4. Stack

5. Binary Search

6. Trees

7. Heap / Priority Queue

8. Backtracking

9. Graphs

10. Dynamic Programming

Final Thoughts