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Practiced Patterns

This document tracks the algorithmic patterns encountered while solving LeetCode problems.

The goal is not to measure mastery, but to record exposure. A pattern is added once it has been implemented in at least one problem. As more exercises are solved, the lists grow, making it easier to identify which techniques have been practiced repeatedly and which still need focused work.

The tables below provide two complementary views:

  • Practiced patterns: groups problems by the primary algorithmic technique they exercise.
  • Patterns by problem: records the main pattern(s) used to solve each individual problem.

Patterns

Pattern Notes Problems
Array / Linear Scan The "default" pattern. Many Easy problems are just one pass over an array or string. #01
#13
#14
#28
In-place Edit Modify the existing container while maintaining a write index #26
#27
String String-specific manipulation/search. #14
String Search Find the first occurrence of a pattern within a larger string. #28
Linked List Separate from Two Pointers because the mechanics differ. #21
Hash Table Dictionaries, sets, counting, lookups. #01
#13
#20
Stack LIFO problems. #20
Queue FIFO problems.
Two Pointers Left/right, fast/slow, read/write. #09
#21
#26
#27
Sliding Window Dynamic intervals.
Binary Search Search over ordered space. #14
#35
Tree DFS/BFS Tree traversals. #
Graph DFS/BFS Graph traversals.
Heap / Priority Queue Top-K, scheduling, streaming.
Backtracking Search with undo.
Dynamic Programming Memoization/state transitions.
Greedy Locally optimal choices.
## Problems
Problem Primary Pattern Secondary Pattern
#1 Two Sum HashMap Array
#9 Palindrome Number Two pointers (or half-reversal) Math
#13 Roman to Integer HashMap Linear scan
#14 Longest Common Prefix String Linear scan
#20 Valid Parentheses Stack HashMap (matching pairs)
#21 Merge Two Sorted Lists Two pointers Linked List
#26 Remove Duplicates from Sorted Array Two pointers In-place array
#27 Remove Element Two pointers In-place filtering
#28 Find First Occurrence String Search Linear scan
#35 Search Insert Position