0113 problems
Sliding Window
Turn nested scans of subarrays and substrings into one linear pass by maintaining a window that grows and shrinks as you go.
The pattern library
The 20 coding interview patterns
Most interview questions are variations on a small set of patterns. Learn each one, then let your daily reviews keep it in memory.
- 0215 problems
Two Pointers
Walk a sequence from both ends (or at two speeds) to find pairs and partitions without a second nested loop.
- 038 problems
Fast & Slow Pointers
Detect cycles and find midpoints in linked structures with two pointers moving at different speeds.
- 0415 problems
Binary Search
Halve the search space every step - on sorted arrays, but also on answer ranges you can test monotonically.
- 059 problems
Breadth-First Search
Explore level by level with a queue - the go-to for shortest paths in unweighted graphs and grids.
- 0615 problems
Depth-First Search
Follow each branch to the end before backtracking - the backbone of tree traversal, connectivity, and exhaustive search.
- 076 problems
Prefix Sums
Precompute running totals so any range query collapses to one subtraction.
- 0812 problems
Linked List Manipulation
Reverse, splice, and rewire lists in place with careful pointer bookkeeping and a dummy head.
- 0912 problems
Backtracking
Build candidates one choice at a time and undo dead ends - permutations, combinations, and constraint puzzles.
- 1016 problems
Dynamic Programming
Break problems into overlapping subproblems and cache the answers, bottom-up or top-down.
- 1113 problems
Greedy
Take the locally best choice at each step - and know how to argue it stays globally optimal.
- 128 problems
Heap / Top-K
Keep the K best items in a heap so you never sort more than you need.
- 136 problems
Intervals
Sort by start, then sweep: merging, inserting, and detecting overlaps in interval lists.
- 1410 problems
Monotonic Stack
A stack that stays sorted answers 'next greater element' questions in one pass.
- 156 problems
Union-Find
Track connected components with near-constant-time merges - cycles, islands, and network connectivity.
- 166 problems
Trie
A prefix tree that makes word lookup, autocomplete, and prefix counting linear in word length.
- 174 problems
Topological Sort
Order tasks with dependencies using indegrees and a queue - and detect when no valid order exists.
- 188 problems
Bit Manipulation
XOR tricks, masks, and shifts that turn set logic into constant-time integer operations.
- 1916 problems
Matrix Traversal
Spirals, diagonals, islands, and flood fill - moving through 2D grids without losing your place.
- 203 problems
Two Heaps
A max-heap and min-heap balanced against each other give you a running median in log time.