Wei Yi Chung

Adjacent comparisons and walking monotonic runs (uphill, downhill, hills and valleys), keeping prefix / suffix maximums and sums in one pass, enumerating pairs in small ranges, and breaking ties with tuple keys

Common bit operations, AND only removes bits while OR only adds them, finding adjacent ones with n & (n » 1), and all-ones numbers 2^k - 1

Split the value range into n - 1 buckets; the largest gap can never fall inside one bucket, so keeping each bucket’s minimum and maximum finds the maximum adjacent difference in linear time

Counting and frequency of frequencies, using the array itself as a hash table (sign marking), counting several keys at once (rows, columns, diagonals), incremental counts from a fixed left end, and grouping by a normalized key

Dummy heads, reversal (whole list, a range, half), fast and slow pointers for middles and cycles, two pointers n apart, pairwise swaps, two lists meeting, and copying a list with random pointers via a hash map

Transposing and rotating, converting between 2D indices and a 1D sequence, neighbors and diagonals, and spiral traversal with four boundaries

Point updates with range queries: a Fenwick tree (binary indexed tree) jumps by i & -i, and a segment tree recursively halves the range; both take O(log n) per operation

Combining conditions in SELECT … WHERE, and NULL’s three-valued logic: any comparison with NULL is not true, so use IS NULL

Simulation by building a list, parsing by trying every pattern at each position, all substrings of length k, counting by character class, checking balanced substrings from a fixed start, and splitting into words

The choose, recurse, undo template for subsets, permutations, constrained placement, grid search, and pruning

Connected components, grid flood fill, multi-source BFS, state-space BFS, shortest cycles, and recursion depth in DFS

Lower / upper bound, binary search on the answer, k-th smallest, integer roots, and why the two interval styles must not be mixed