DSA Patterns (Codolio Roadmap)

Every problem on the Codolio / Neetcode 150 / Striver A2Z / Blind 75 sheet falls into about 18 recurring patterns. Master the pattern -> recognise it in seconds -> apply the template.

Use the sidebar to jump to any pattern.

The 18 Patterns at a Glance

#	Pattern	When you see…	Classic problems
1	Sliding Window	longest/shortest subarray with constraint	Max Sum Subarray of Size K, Longest Substring w/o Repeats
2	Two Pointers	sorted array -> find pair / reverse	Two Sum (Sorted), Valid Palindrome
3	Fast & Slow	linked list cycle / find middle	Linked List Cycle, Middle of LL
4	Merge Intervals	overlapping time-slots / ranges	Merge Intervals, Insert Interval
5	Cyclic Sort	numbers in [1..N] in O(1) space	Missing Number, Find Corrupt Pair
6	In-place Reversal	reverse linked list in chunks	Reverse LL, Reverse K-Group
7	BFS	shortest path / level-order	Level Order, Rotting Oranges
8	DFS	path exists / connectivity / topo	Number of Islands, Clone Graph
9	Binary Search	sorted array / monotone function	Search in Rotated Array, Koko Bananas
10	Topological Sort	task scheduling dependency	Course Schedule, Alien Dictionary
11	Backtracking	”all” combinations / permutations	Subsets, N-Queens, Word Search
12	Dynamic Programming	overlapping subproblems	Coin Change, LCS, Edit Distance
13	Greedy	activity selection / interval scheduling	Jump Game, Gas Station
14	Bit Manipulation	XOR to find unique / set bits	Single Number, Counting Bits
15	Heap / Priority Queue	top-K / running median	Kth Largest, Median from Stream
16	Trie	prefix search / autocomplete	Implement Trie, Word Search II
17	Monotonic Stack	”next greater element”	Daily Temperatures, Largest Rectangle
18	Prefix Sum	subarray sum queries	Subarray Sum Equals K, Range Sum

How to Use This Section

Read the What is it? and Data Flow diagrams until you can sketch them on a whiteboard.
Write out the template code (slow, sloppy is fine).
Solve the Classic problems linked in the table.
Time yourself against the Pitfalls — the most common miss is forgetting to update one pointer or shrink the window.

Mnemonic: every pattern has a one-line trigger. Memorise the trigger, not the steps.

Flow: From Pattern Recognition to Solution

+-------------------+      +-------------------+      +-------------------+
|  Read problem     | ---> |  Identify pattern | ---> |  Apply template   |
|  text & examples  |      |  from triggers    |      |  & tweak          |
+-------------------+      +-------------------+      +-------------------+
                                                            |
                                                            v
                                                    +-------------------+
                                                    |  Verify with edge |
                                                    |  cases + dry run  |
                                                    +-------------------+

Universal Interview Heuristic

If you cannot name the pattern within 5 minutes of reading the problem:

Re-read the input/output carefully.
List the constraint keywords (“sorted”, “subarray”, “in-place”, “O(1) space”).
Check the elimination list: any clue pointing to TWO POINTERS, BINARY SEARCH or HASHMAP?
If still stuck, fall back to BRUTE FORCE first (your first 15 minutes) then optimise.