Technology
24 min read
Vertices and edges, adjacency lists versus matrices, breadth-first and depth-first search, connected components, grids as graphs, and basic cycle detection in undirected and directed graphs, with complexity in terms of V and E.
Technology
20 min read
Binary numbers, the bitwise operators and Java's signed-integer behaviour, then the reasoned uses: parity, powers of two, counting bits, XOR, masks and subsets, plus GCD, modular arithmetic, fast exponentiation and the sieve of Eratosthenes.
Technology
22 min read
When picking the best-looking option at every step is actually correct, and when it is not. Learn to justify a greedy strategy, hunt for counterexamples, and solve activity selection, merge intervals, meeting rooms and jump game.
Technology
19 min read
Answer range-sum queries in constant time with a running-total array, count subarrays with a given sum using a hash map, compute product-except-self, handle 2D regions, and apply many range updates at once with a difference array.
Technology
18 min read
Fixed-size and variable-size windows over arrays and strings, with running sums, counters and frequency maps. Learn the expand-and-shrink template, why it runs in O(n), and when negative numbers break it.
Technology
18 min read
Replace nested loops with two indices that move in a controlled way. Learn the opposite-ends and same-direction patterns, why each pointer move is safe, and solve pair sum, container with most water, 3Sum and in-place deduplication.
Technology
22 min read
Linear and binary search done right (invariants, boundaries and overflow-safe midpoints), lower and upper bound, searching on the answer, the main sorting algorithms and their trade-offs, and how to sort anything in Java with comparators.
Technology
21 min read
How a heap keeps the smallest or largest item ready in O(1), how insert and remove work in O(log n), how to use Java's PriorityQueue, and how to solve top-K, k-th largest and merge-k-sorted problems. Includes how a heap differs from a BST.
Technology
22 min read
Tree vocabulary, the four traversals (preorder, inorder, postorder, level-order), recursive and iterative code, and binary search trees: search, insert, and the right way to validate one.
Technology
24 min read
Learn to think recursively: base cases, the call stack and recursion trees. Then use backtracking (choose, explore, undo) to generate subsets, permutations and combinations, avoid duplicates, prune the search and solve N-Queens.
Technology
20 min read
How hash tables find things almost instantly, what collisions are, how to use HashMap and HashSet in Java, and the patterns that solve duplicates, two-sum, grouping and counting problems. Includes the equals and hashCode contract.
Technology
19 min read
Last-in-first-out and first-in-first-out, explained with real examples. Learn the right Java collections (ArrayDeque, not Stack), then solve bracket matching, round-robin scheduling and next-greater-element with a monotonic stack.
Technology
20 min read
Build singly and doubly linked lists from scratch in Java, learn to reason about pointer changes step by step, and solve reversal, middle-finding, merging and cycle detection with clear diagrams and dry runs.
Technology
17 min read
How Java strings really work: characters and codes, comparison, immutability and StringBuilder, then the classic problems (palindromes, anagrams, run-length encoding) plus honest notes on Unicode.
Technology
20 min read
How arrays work in memory, why indexing is instant but inserting is slow, and how to solve the classic array problems: maximum, frequency counting, in-place rotation, moving zeroes and subarray sums.
Technology
19 min read
Big O explained by counting steps: constant, logarithmic, linear, n log n, quadratic and exponential growth, best, average and worst cases, auxiliary and recursion-stack space, with dry runs you can check yourself.
Technology
21 min read
Only the Java you need for data structures and algorithms: types and overflow, loops, methods, arrays, strings, a small class, input and output, and the mistakes that trip up almost every beginner.
Technology
17 min read
How to turn a vague problem statement into a clear plan: pin down inputs, outputs and constraints, list edge cases, write pseudocode, build a brute-force answer first and then improve it.
