Which DSA pattern? Spot it from the question

Three questions to ask first

What is the input? (list, tree, intervals, grid or graph, words, or an array or string)
What exactly do they ask for? (all of them, how many, best, k-th, contiguous, a position…)
What are the limits? (n ≤ 20 means exponential is fine; 10⁵ means O(n log n) or better; 'O(1) space' or 'O(log n)' rule out options)

Step 1 · Look at the input

flowchart TD
  I{"What is the input?"}
  I -->|"a linked list"| LL["Linked List<br/>dummy node · prev/curr/next · fast & slow"]
  I -->|"a tree"| T{"levels or 'closest'?"}
  T -->|"yes"| TB["Trees: BFS with a queue"]
  T -->|"no"| TD2["Trees: DFS recursion"]
  I -->|"[start, end] pairs"| IV["Intervals: sort by start"]
  I -->|"a grid, or nodes and edges"| G{"edges with different costs?"}
  G -->|"yes"| AG["Advanced Graphs: Dijkstra"]
  G -->|"no"| GR["Graphs: DFS/BFS · topological sort · union-find"]
  I -->|"many words, prefixes"| TR["Tries"]
  I -->|"an array or a string"| NEXT["→ Step 2: look at the question"]

Start with the shape of the input. Lists, trees, intervals, graphs and word sets almost choose the pattern for you.

Step 2 · Arrays and strings: look at the question

flowchart TD
  Q{"What are they asking for?"}
  Q -->|"every combination / permutation"| BT["Backtracking"]
  Q -->|"number of ways, or min / max with choices"| DP{"greedy breaks on a small example?"}
  DP -->|"yes"| DPY["Dynamic Programming"]
  DP -->|"no, one safe best move"| GRD["Greedy"]
  Q -->|"best contiguous subarray / substring"| SW["Sliding Window"]
  Q -->|"a pair or triplet, sorted data"| TP["Two Pointers"]
  Q -->|"a position or smallest value that works, O(log n)"| BS["Binary Search"]
  Q -->|"k-th, top k, a stream"| HP["Heap"]
  Q -->|"next greater, matching brackets"| ST["Stack"]
  Q -->|"seen before, counts, groups"| HM["Arrays & Hashing"]
  Q -->|"pairs cancel, single bits, no + allowed"| BIT["Bit Manipulation"]
  Q -->|"rotate, spiral, digits"| MG["Math & Geometry"]

For arrays and strings, the wording of the question is the clue. Match the phrase, then confirm with the pattern's 3-second gut check.

The drill

Read what a problem asks and pick its pattern. A right answer brings the card back later (1, 3, 7, 14, then 30 days); a wrong one brings it back tomorrow. Each day: the cards that are due, plus five new ones.

Loading the drill…

Patterns people confuse

Pattern	Often confused with
Arrays & Hashing	Two Pointers, Sliding Window, Heap / Priority Queue
Two Pointers	Sliding Window, Binary Search, Arrays & Hashing
Sliding Window	Two Pointers, Arrays & Hashing, 1-D Dynamic Programming
Stack	Two Pointers, Heap / Priority Queue, Greedy
Binary Search	Two Pointers, Heap / Priority Queue, Greedy
Linked List	Two Pointers, Arrays & Hashing, Stack
Trees	Graphs, Backtracking, Binary Search
Heap / Priority Queue	Binary Search, Arrays & Hashing, Greedy
Backtracking	1-D Dynamic Programming, Graphs, Trees
Tries	Arrays & Hashing, Backtracking, Trees
Graphs	Trees, Advanced Graphs, 2-D Dynamic Programming
Advanced Graphs	Graphs, Heap / Priority Queue, 1-D Dynamic Programming
1-D Dynamic Programming	Greedy, Backtracking, 2-D Dynamic Programming
2-D Dynamic Programming	1-D Dynamic Programming, Graphs, Backtracking
Greedy	1-D Dynamic Programming, Sliding Window, Intervals
Intervals	Greedy, Heap / Priority Queue, Two Pointers
Math & Geometry	Arrays & Hashing, 2-D Dynamic Programming, Bit Manipulation
Bit Manipulation	Math & Geometry, Arrays & Hashing, Greedy

Which pattern? Spot it from the question

Three questions to ask first

Step 1 · Look at the input

Step 2 · Arrays and strings: look at the question

The drill

Patterns people confuse