Topic-wise Questions

Arrays & Hashing

Start here. Every later pattern reuses these moves: in-place writes, counting with hash maps and prefix sums.

Strings

Arrays of characters with their own tricks: parsing, comparing and expanding around centres.

Two Pointers

Two indices moving toward each other turn many O(n²) scans on sorted data into O(n).

Sliding Window

Grow and shrink a window to answer subarray and substring questions in one pass.

Stack & Queue

Last in, first out: match pairs and keep track of what is still waiting for an answer.

Binary Search

Halve the search space every step, on sorted arrays and on the answer itself.

Linked List

Pointer rewiring: dummy heads, fast and slow pointers, and in-place reversal.

Recursion & Backtracking

Explore every choice, undo it, and try the next: subsets, permutations and searches.

Binary Trees

Recursion on a shape: traversals first, then problems that combine answers from both children.

Binary Search Trees

Use the ordering: left < node < right turns many tree problems into binary search.

Heaps & Priority Queues

Keep the best k things at hand: top-k, scheduling and running medians.

Greedy

Make the locally best choice, and be able to say why it never hurts.

Intervals

Sort by start or end, then sweep.

Graphs

Model the problem as nodes and edges, then BFS or DFS: islands, components, cycles and ordering.

Advanced Graphs

Weighted edges: shortest paths, spanning trees and bridges.

1-D Dynamic Programming

Reuse answers to smaller versions of the problem: recursion, then memo, then a table.

2-D Dynamic Programming

Two changing quantities: grid positions, or an index plus a budget.

DP on Strings

Compare two strings, or a string with itself, one character at a time.

Advanced DP

State machines and interval DP: the hardest patterns, best learned last.

Tries & String Algorithms

Prefix trees for word problems, and linear-time string matching.

Bit Manipulation

XOR, masks and shifts: small tricks that show up everywhere.

Math & Geometry

Number tricks and coordinate problems that interviews still like.