Array

LeetCode 452 note: sort by end and shoot at the end; only a balloon starting after the arrow needs a new one, and touching ends count as overlapping

LeetCode 1011 note: binary search on the answer over [max(weights), sum(weights)], checking the day count by filling greedily in order

LeetCode 4099 note: the swaps needed equal the inversion count; every locked position some element must cross has to be unlocked, so count those, compare with k, and add the inversions counted with merge sort

LeetCode 4097 note: only merge the fixed pairs (0,1), (2,3), …, stepping by 2, not every adjacent pair

LeetCode 889 note: keep the current path on a stack; when its top equals the next postorder value that subtree is complete and is popped; new nodes go left first

LeetCode 1186 note: Kadane with one more state, ending here with no deletion or with one deletion; update the answer at every step

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

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

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

LeetCode 775 note: turn a counting problem into an existence check and solve it in O(n)

LeetCode 305 note: each new land cell adds an island, then merges with neighboring land, subtracting 1 per successful merge; skip repeated positions

LeetCode 2584 note: coprime products ⇔ no prime factor in both parts; record each prime’s last index, extend reach left to right, and split after i when reach == i