Neetcode 250 (Java)

Arrays & Hashing

Frequency counting, grouping, and lookups with hash maps and sets.

Scan from both ends or with two indices instead of nested loops.

Maintain a moving range over an array or string instead of recomputing it.

Last-in, first-out problems: matching pairs, monotonic stacks, and expression evaluation.

Halve the search space on sorted data or on a monotonic answer.

Pointer manipulation, fast and slow pointers, and designs built on linked lists.

Binary tree recursion, traversals, and binary search trees.

Prefix trees for fast prefix lookup, wildcard search, and searching many words at once.

Repeatedly pull out the smallest or largest item, or keep only the top k.

Explore choices depth first, undo them, and prune dead ends.

Grid and graph traversal with DFS, BFS, topological sort, and union-find.

Shortest paths, minimum spanning trees, Eulerian paths, and topological ordering.

Break a problem into overlapping subproblems indexed by one variable.

Grid, string-pair, and interval DP with two changing indices.

Make the locally best choice at each step and prove it is safe.

Sort by start or end and sweep to merge, insert, or count overlaps.

Matrix transformations, number manipulation, and coordinate tricks.

XOR tricks, shifts, and masks on integers.