leetcode/docs/content/02-guides/01-coding/02-recognition.mdx

---
title: 'Recognition'
description: 'There are only 6 structural families. Every pattern is a traversal or maintenance strategy over one of them.'
---


## Family 1 — Contiguous Linear (arrays, strings)
*Structure property: O(1) index access, order is information.*

| Pattern | What it really is | Recognition triggers |
|---|---|---|
| **Two Pointers** (7) | Converging/parallel index walk that prunes the O(n²) pair space using order | Sorted input · pair/triplet with target · palindrome · in-place partition |
| **Sliding Window** (5) | Two pointers + incremental state between them (Two Pointers with a payload) | "Longest/shortest substring/subarray with property X" · contiguous constraint |
| **Prefix Sum** (2) | Precomputed cumulative state → O(1) range queries | "Sum of range" · "subarray sum equals K" · repeated interval aggregation |
| **Binary Search** (5) | Halving over ANY monotonic predicate — not just sorted arrays | Sorted/rotated input · "minimize the maximum" · answer space is monotonic (Koko) |
| **Matrices** (3) | 2D index arithmetic; often a graph in disguise | Grid traversal/rotation/spiral · if cells have neighbors → it's Family 5 |

**Systems anchor:** prefix sum is a materialized view; sliding window is stream processing with an eviction policy.

## Family 2 — Nodal Linear (linked lists)
*Structure property: O(1) splice, no random access. Everything is pointer surgery.*

| Pattern | What it really is | Recognition triggers |
|---|---|---|
| **Linked List** (5) | Three moves cover it all: fast/slow runners, reversal, dummy-head splicing | Cycle detection · middle/Nth-from-end · reverse/reorder · merge |

**Systems anchor:** LRU Cache = hashmap + doubly-linked list — the one hybrid that appears constantly (it's a cache eviction index).

## Family 3 — Auxiliary Ordering Structures (stack, heap)
*Structure property: you don't traverse them — you maintain them while traversing something else.*

| Pattern | What it really is | Recognition triggers |
|---|---|---|
| **Stack** (5) | LIFO memory of "unresolved" items; monotonic stack = discard dominated candidates | Matching/nesting (parens) · "next greater element" · "previous smaller" · expression eval |
| **Heap** (5) | Partial order — cheap access to extremum without full sort | "Kth largest/smallest" · "top K" · merge K streams · running median (two heaps) |

**Systems anchor:** monotonic stack ≈ log compaction (drop entries superseded by newer state); heap ≈ priority scheduling / Dijkstra's frontier / timer wheels.

## Family 4 — Hierarchical (trees, tries)
*Structure property: recursive shape — every node roots a subtree. Solutions are recurrences.*

| Pattern | What it really is | Recognition triggers |
|---|---|---|
| **DFS on trees** (part of 13) | Post/pre/in-order recursion; answer composed from subtree answers | "Max depth / diameter / LCA / validate" · anything defined recursively |
| **BFS on trees** (part of 8) | Level-order via queue | "Level by level" · "right side view" · minimum depth |
| **Trie** (2) | Character-indexed tree; shared prefixes = shared paths | Prefix search · autocomplete · word dictionaries · many-string matching |
| **BST specifically** | Sorted array in tree form — in-order traversal IS sorted order | "Kth smallest in BST" · range queries · validate |

**Systems anchor:** trie ≈ radix routing table / IPLD path resolution; BST invariant ≈ index B-tree.

## Family 5 — Relational (graphs)
*Structure property: arbitrary edges; must track visited. Trees are just acyclic connected graphs — same traversals, plus bookkeeping.*

| Pattern | What it really is | Recognition triggers |
|---|---|---|
| **DFS on graphs** (13) | Exhaustive reachability; recursion + visited set | Islands/regions/flood fill · connectivity · cycle detection (directed: 3-color) |
| **BFS on graphs** (8) | Frontier expansion → **shortest path in unweighted graphs, always** | "Minimum steps/moves" · multi-source spread (rotting oranges) · word ladder |
| **Graphs** (6): Topological sort | DFS post-order reversed / Kahn's in-degree queue over a DAG | Dependencies · prerequisites · build order · "valid ordering exists?" |
| **Graphs** (6): Union-Find | Incremental connectivity without traversal | Dynamic "same component?" queries · redundant edge · counting components |

**Systems anchor:** topo sort = dependency resolution (your Turborepo build graph); union-find = partition detection; BFS = gossip propagation. A matrix with neighbor-moves is ALWAYS this family, not Family 1.

## Family 6 — Decision-Space (implicit graphs you build while exploring)
*Structure property: nodes are STATES, not data. The structure doesn't exist until you walk it.*

| Pattern | What it really is | Recognition triggers |
|---|---|---|
| **Backtracking** (6) | DFS over the state tree with undo; exhaustive enumeration | "All subsets/permutations/combinations" · constraint satisfaction · "generate all valid X" |
| **Dynamic Programming** (10) | Backtracking + memoization — overlapping subproblems collapse the exponential tree | "Count ways" · "min/max cost to reach" · choices with shared substructure · optimal substructure |
| **Greedy** (5) | DP where the local optimum provably suffices — no memo needed | Sort-then-commit works · exchange argument holds · intervals scheduling · jump game |

**Systems anchor:** DP memo = idempotency cache over a deterministic state machine; backtracking = saga with compensating undo; greedy = eventually-consistent local decisions that provably converge.

---

## The Recognition Decision Tree (run this in the first 2 minutes — rubric axis #1)

1. **Contiguous array/string, no neighbor semantics?** → Family 1. Sorted or monotonic answer space → Binary Search. Pair/window constraint → Two/Sliding. Range sums → Prefix.
2. **Nodes with next-pointers?** → Family 2. Reach for fast/slow or dummy-head before anything clever.
3. **Need "next greater," nesting, or top-K while scanning?** → Family 3 auxiliary structure on top of whatever you're traversing.
4. **Recursive shape (tree/prefix data)?** → Family 4. Ask: does the answer compose from subtrees (DFS) or from levels (BFS)?
5. **Arbitrary connections, grids with movement, dependencies?** → Family 5. Unweighted shortest = BFS. Ordering over DAG = topo. Dynamic connectivity = union-find. Otherwise DFS.
6. **Enumerate or optimize over CHOICES?** → Family 6. All solutions → backtracking. Overlapping subproblems → DP. Provable local choice → greedy.

## The Escalation Lattice (when stuck, patterns upgrade along these edges)

- Two Pointers —*add state*→ Sliding Window —*add aggregation*→ Prefix Sum
- Brute-force scan —*monotonic predicate*→ Binary Search on answer
- Tree DFS —*add visited set*→ Graph DFS —*add undo*→ Backtracking —*add memo*→ DP —*add proof*→ Greedy
- Sort —*need only extremum*→ Heap —*need running extremes*→ Monotonic Stack
- BFS —*add weights*→ Dijkstra (= BFS with a heap: Families 3+5 composed)

Memorize the lattice, not 16 patterns. In the interview, naming the family out loud ("this is a state-space problem, so backtracking or DP depending on overlap") IS the recognition signal the rubric scores.