Pattern Wise 300: Java Interview Problems

01

Array Manipulation

10

Instinct "In place", "O(1) extra space" or "one pass" over an array? Think write pointer, reversal, prefix/suffix passes or using indices as storage.Memory hack Read fast, write slow, reverse to rotate, and let the index be the hash.Open the Array Manipulation guide →

02

String Manipulation

6

Instinct Reading, comparing or rebuilding text character by character? Walk indices on a char[] and build output with a StringBuilder, never with + in a loop.Memory hack Chars in an array, pointers on the ends, builder at the back - strings are just arrays wearing a costume.Open the String Manipulation guide →

03

Hashing & Counting

13

Instinct Need "have I seen this before?", "how many times?" or "group things that look alike"? Trade memory for O(1) lookups with a HashMap, HashSet or int[26].Memory hack Don't search, remember - every element checks in at the front desk, and the desk answers instantly.Open the Hashing & Counting guide →

04

Two Pointers

5

Instinct Sorted input, a pair/triple hitting a target, or something symmetric? Put one pointer at each end and move the one that can only improve things.Memory hack Squeeze from both ends - every step throws away a whole row of the brute-force grid.Open the Two Pointers guide →

05

Sliding Window

10

Instinct Longest, shortest or count of contiguous subarrays/substrings meeting a rule? Grow a window on the right, shrink it on the left, never step back.Memory hack Move like a caterpillar - the head stretches forward to grab, the tail pulls up only when the body breaks the rule.Open the Sliding Window guide →

06

Prefix Sum

5

Instinct Repeated range-sum queries, or "count subarrays whose sum equals / divides K" with negatives allowed? Store running totals and subtract two of them.Memory hack Any range is two prefixes apart - sum(i..j) = P[j+1] - P[i], so remember every P you've passed.Open the Prefix Sum guide →

07

Kadane's Running Best

4

Instinct Best sum or product of a contiguous subarray in one pass? At each index choose - extend the run ending here or restart - and track the global best.Memory hack Carry the bag only while it helps - a negative past is dead weight, drop it and restart here.Open the Kadane's Running Best guide →

08

Matrix Traversal

5

Instinct A 2D grid you must walk in a fixed geometric order (spiral, diagonal, layer, rotate) or update in place - think boundaries, direction vectors and index maths.Memory hack Walk the walls, then shrink the room - four fences (top, bottom, left, right) close in after every lap.Open the Matrix Traversal guide →

09

Bit Manipulation

7

Instinct Pairs that cancel, counting 1s, powers of two, subsets as numbers, or 'no + or - allowed' - drop to binary and use XOR, AND, OR and shifts.Memory hack XOR is a forgetful twin detector - see something twice and it vanishes; n & (n-1) snips off the last lit bulb.Open the Bit Manipulation guide →

10

Bucket & Counting Sort

3

Instinct Keys live in a small, known range (letters, frequencies 1..n, bounded values) - index an array by the key instead of comparing, and get O(n).Memory hack Don't compare, just file it - every value walks straight to its own pigeonhole, then you read the shelves in order.Open the Bucket & Counting Sort guide →

11

Linked List Techniques

14

Instinct Nodes joined by next pointers and the task is to reverse, splice, merge, reorder or remove in place - grab a dummy head and rewire pointers carefully.Memory hack Save next, flip, step - never let go of a rope before you've grabbed the next one.Open the Linked List Techniques guide →

12

Fast & Slow Pointers

3

Instinct A next-step function you can follow (list nodes, digit-square sums, array-as-pointers) and you need a cycle, its entry, or the middle in O(1) space.Memory hack Tortoise and hare - on a loop track the hare laps the tortoise; restart the tortoise at the gate and they meet at the loop's door.Open the Fast & Slow Pointers guide →

13

Stack

8

Instinct Things resolve in reverse order of arrival (brackets, adjacent pairs, nested expressions) - push now, pop when its partner shows up.Memory hack Last in, first out - like a pile of plates, you only ever touch the one on top; every closer pops its opener.Open the Stack guide →

14

Monotonic Stack

6

Instinct Each element needs its nearest bigger or smaller neighbour (next warmer day, span, widest bar) - keep a sorted stack of waiting indices.Memory hack Tall people block the view - a newcomer kicks out everyone shorter waiting behind, and each kicked-out person just found their answer.Open the Monotonic Stack guide →

15

Queue & Monotonic Deque

7

Instinct Arrival order matters, or you need the max/min of a moving window - reach for a FIFO queue, or a deque of indices kept in sorted order.Memory hack Old ones leave by the front door, weak ones get bounced out the back - so whoever stands at the front is always the champion.Open the Queue & Monotonic Deque guide →

16

Data Structure Design

8

Instinct Implement a class whose methods must all hit a time bound (often O(1)) - write each method's target cost, then glue a hash map to a structure that keeps order.Memory hack Finder plus keeper - a hash map finds it fast, a list/heap/array keeps the order, and every method updates both.Open the Data Structure Design guide →

17

Binary Search

10

Instinct Sorted data, a rotated array, or find the min/max X that works where a yes/no test flips only once - halve the search space every step.Memory hack Find the fence - every candidate reads NO NO NO YES YES, and binary search just walks to where the fence stands.Open the Binary Search guide →

18

Recursion & Divide and Conquer

8

Instinct If the answer for n items can be built from the same problem on halves (or smaller pieces) plus a cheap combine step - split, recurse, merge.Memory hack Split it, trust it, stitch it - nail the base case, take the leap of faith on smaller calls, then glue their answers together.Open the Recursion & Divide and Conquer guide →

19

Partitioning & Quickselect

2

Instinct Need the k-th smallest/largest, an unordered top-k, or values grouped into buckets in place - partition around a pivot and chase only one side.Memory hack Pivot, partition, pick a side - quickselect is quicksort's lazy cousin that only chases the half holding k.Open the Partitioning & Quickselect guide →

20

Backtracking

8

Instinct Return ALL combinations, permutations, partitions or placements - or ask if any arrangement works - build a candidate step by step and undo each choice.Memory hack Choose, explore, un-choose - walk the decision tree and snip a branch the moment it can no longer succeed.Open the Backtracking guide →

21

Greedy

7

Instinct An optimisation where one obvious local rule (earliest end, farthest reach, biggest first) never needs undoing - commit to it, then try hard to break it.Memory hack Grab the best bite now and never look back - but only once a counterexample hunt fails to make you regret it.Open the Greedy guide →

22

Intervals & Line Sweep

7

Instinct See [start, end] pairs, meetings, bookings or the word "overlap"? Sort by an endpoint, then sweep left to right carrying one running fact.Memory hack Sort, then sweep - start time to merge, end time to choose, +1/-1 events to count the crowd.Open the Intervals & Line Sweep guide →

23

Tree BFS (Level Order)

5

Instinct The answer talks about levels, rows, depth order, "closest to the root" or what you see from a side? Queue the tree and process it one level at a time.Memory hack Freeze the size, drain the row - read n = queue.size() before the loop and one level never bleeds into the next.Open the Tree BFS (Level Order) guide →

24

Tree DFS

20

Instinct If the answer for a node can be built from the answers of its left and right subtrees, write one recursive function and trust it on the children.Memory hack Ask the kids, then decide - each child reports up, the parent combines, and a global variable keeps the best "bend" seen so far.Open the Tree DFS guide →

25

Binary Search Tree & In-order

6

Instinct The tree is a BST? Its in-order walk is a sorted list and every node splits the search space - think "sorted array in tree form", not "visit everything".Memory hack Left is less, right is more, in-order reads the sorted score - and every node lives inside a (low, high) fence set by its ancestors.Open the Binary Search Tree & In-order guide →

26

Ordered Set (TreeMap)

4

Instinct Keys keep arriving or leaving and you must ask "what is the nearest key below/above x?" or "current min/max" each time - reach for TreeMap/TreeSet.Memory hack A HashMap with a sense of direction - floor looks down, ceiling looks up, and both answer in log time while the data keeps changing.Open the Ordered Set (TreeMap) guide →

27

Trie

6

Instinct Many words, shared prefixes, and questions like "starts with", autocomplete, wildcard match or "search a board for all these words" - build a prefix tree.Memory hack Words share their beginnings, so store each beginning once - walk one letter per step, and an end flag marks where a real word stops.Open the Trie guide →

28

Heap & Top-K

6

Instinct "K largest / smallest / closest / most frequent", or "always take the best available next"? Keep a heap - size k for top-K, or a ready-queue for scheduling.Memory hack Guard the top-K club with a bouncer - a min-heap of size k whose root is the weakest member; anyone stronger kicks them out.Open the Heap & Top-K guide →

29

Two Heaps

3

Instinct When you need the middle of a changing collection, or best-of-one-pile fed by another pile, split it into a max-heap low half and a min-heap high half.Memory hack Small fries in a max-heap, big fish in a min-heap; the median lives on the shoreline between them.Open the Two Heaps guide →

30

K-Way Merge

4

Instinct Several sorted lists, rows or streams and you need the global order, the k-th smallest or a range: keep one cursor per list in a min-heap.Memory hack Line up the heads, crown the smallest, then let its next-in-line step up.Open the K-Way Merge guide →

31

Fenwick & Segment Tree

2

Instinct Range sums, mins or counts asked again and again while values keep changing: prefix sums break, so reach for a Fenwick or segment tree.Memory hack Prefix sums freeze, trees breathe: climb by the last set bit (i & -i) for Fenwick, split in halves for segment trees.Open the Fenwick & Segment Tree guide →

32

Graph DFS

10

Instinct Connected regions, all paths, reachability, cloning or 2-colouring a graph or grid: dive from a node, mark it, recurse into unvisited neighbours.Memory hack Mark before you march: go deep, paint as you go, and back up only when every door is already painted.Open the Graph DFS guide →

33

Graph BFS

6

Instinct Minimum moves, fewest steps or shortest transformation where every step costs the same: spread outward level by level with a queue.Memory hack Ripples in a pond: every ring is one step farther, so the first ring that touches the target is the answer.Open the Graph BFS guide →

34

Topological Sort

4

Instinct Tasks with prerequisites, "X must come before Y", or an order hidden in comparisons: build a directed graph and peel off nodes with zero in-degree.Memory hack No prereqs? Off you go. Each finished task frees its followers; if someone is never freed, there is a cycle.Open the Topological Sort guide →

35

Union-Find

4

Instinct Items get grouped by "connected", "same as" or "shares a" links and you keep asking which group something is in: merge sets with Union-Find.Memory hack Find your boss, merge the bosses; squash the chain (path compression) and hang the short tree under the tall one (rank).Open the Union-Find guide →

36

Shortest Paths

5

Instinct Weighted edges plus "minimum cost, time or effort to reach" - grab a min-heap and always expand the cheapest unsettled node next (Dijkstra).Memory hack Pop the cheapest, skip the stale, relax the rest - Dijkstra is a ripple in a pond that touches near shores before far ones.Open the Shortest Paths guide →

37

MST & Euler Paths

3

Instinct "Connect every point for the least total cost" means a minimum spanning tree; "use every edge or ticket exactly once" means an Euler path.Memory hack MST - grab the cheapest bridge that joins two separate islands. Euler - wander until stuck, write the stop down on the way back, then reverse.Open the MST & Euler Paths guide →

38

1-D Dynamic Programming

4

Instinct The best answer at position i depends only on a few earlier positions (i-1, i-2, ...) - write dp[i] from its predecessors, then roll it into variables.Memory hack Look back, pick best, roll forward - every step only remembers the last couple of stairs it stood on.Open the 1-D Dynamic Programming guide →

39

Knapsack DP

6

Instinct Choose items to hit or stay under a target sum (can it be done, best value, fewest items, how many ways) - index a DP array by the remaining capacity.Memory hack Once? Walk back. Many? Walk forward. Each item used at most once loops capacity downward; reusable items loop it upward.Open the Knapsack DP guide →

40

Longest Increasing Subsequence

3

Instinct Longest chain where each pick must beat the last (bigger, later, fits inside) - O(n²) best-ending-here DP, or O(n log n) tails plus binary search.Memory hack Patience solitaire - drop each card on the leftmost pile whose top is not smaller; the number of piles is the LIS length.Open the Longest Increasing Subsequence guide →

41

Grid DP

9

Instinct Grid moves that never loop back (right/down, row to row) plus "count or optimise the paths" - build dp[r][c] from the cells that can step in.Memory hack Every cell asks its parents - a cell's answer comes only from the neighbours that can move into it, so fill cells in the order they arrive.Open the Grid DP guide →

42

String DP

9

Instinct Two strings compared char by char (match, edit, interleave) means dp[i][j] over prefixes; one string's palindromes or splits means dp[l][r] or dp[i].Memory hack Match? Go diagonal. Mismatch? Ask the neighbours. Up drops a char from s, left drops one from t, diagonal uses both.Open the String DP guide →

43

Tree & Graph DP

5

Instinct A node's best answer depends only on its children's answers - post-order DFS that returns a tiny tuple of states per node.Memory hack Kids report up, parent decides - every subtree hands its parent a small report card, and the parent never re-reads the kids' homework.Open the Tree & Graph DP guide →

44

State Machine DP

2

Instinct Each step you are in one of a few modes (holding, sold, cooling down) with rules for switching - keep one DP value per mode and update all of them every step.Memory hack Name the moods, draw the moves, max the arrows - every arrow in your state diagram becomes exactly one line of code.Open the State Machine DP guide →

45

Bitmask DP

3

Instinct n is tiny (about 20 or less) and you must use, visit or assign every item - make the set of used items a bitmask and DP over all 2^n subsets.Memory hack Small n, big choice - let the bits keep score: each item is a light switch and the mask is the whole switch panel.Open the Bitmask DP guide →

46

Digit DP

3

Instinct Count numbers in [L, R] (R up to 10^9 or 10^18) whose digits obey a rule - build the number digit by digit with a "still tight to N" flag.Memory hack Walk the digits on a leash - stay tight to N or break free, and once free you never go back. Remember PTLS - Position, Tight, Leading zero, State.Open the Digit DP guide →

47

Probability DP

3

Instinct Asked for a probability or expected value after random moves? Run DP over (step, state) and split each state's mass across every outcome.Memory hack Pour the probability like water - split it across the moves, let what spills off the board drain away, and the puddle that's left is your answer.Open the Probability DP guide →

48

Math & Geometry

6

Instinct Digits, factors, slopes or distances hide a formula - find the invariant, then guard against overflow, negatives and floating-point error.Memory hack Mod peels, gcd reduces, squares compare - and long saves the day. Peel digits with %10, reduce slopes with gcd, compare squared distances.Open the Math & Geometry guide →

49

String Matching

3

Instinct Find a pattern in a text, or a prefix that is also a suffix, faster than O(n·m) - reach for the KMP prefix function or a rolling hash.Memory hack KMP remembers borders, Rabin-Karp compares fingerprints - neither one re-reads text it has already seen.Open the String Matching guide →

Pattern Wise 300