Sorting

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 314 note: BFS with a column index; BFS is what guarantees top-to-bottom within a column and left-to-right within a row

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

Converging pointers, one pointer per sequence, skipping blanks to compare positions, counting pairs after sorting, and the in-place write pointer

Merge sort templates, where to count in inversion-style problems, the linked list version, and common mistakes

Sort then scan, intervals sorted by right end, left and right passes, pairing and parity invariants, adjacent differences, and “remove what exceeds the average”, each with why greedy is correct

LeetCode 912 note: hand-written merge sort and the two bugs I hit

LeetCode 253 note: a heap of end times for running meetings; half-open intervals need <=

LeetCode 2751 note: sort by position, push right-moving robots, and let each left-moving robot collide until one side is gone

LeetCode 1998 note: numbers in one component can be permuted freely; union each number with its prime factors via SPF, and the array sorts iff each position’s old and new values share a group

LeetCode 2561 note: every value’s total must be even; sort the excess, and each cheap half item swaps with an expensive one at min(its value, 2 × global minimum)

LeetCode 987 note: collect (col, row, val) triples, sort them, and group by column