Array

LeetCode 1464 note: all values are positive, so take the two largest a and b and return (a - 1)(b - 1)

LeetCode 1337 note: soldiers come first in each row, so binary search the first 0 for the count; sort by (count, row index) and take k

LeetCode 334 note: keep the smallest first and the smallest second that has something smaller before it; any number above second completes a triplet

LeetCode 324 note: sort, split into a small half and a large half, reverse each, and interleave with small values at even indices and large ones at odd indices

LeetCode 473 note: the total must be divisible by 4; place sticks from longest to shortest into one of four sides, skipping a side that would overflow or that equals a side already tried

LeetCode 435 note: minimum removals = total minus the maximum kept; sort by right end and greedily keep the one ending earliest

LeetCode 396 note: F(k) and F(k - 1) differ by one formula: F(k) = F(k - 1) + sum - n × the element moved to the front

LeetCode 313 note: generalizing LC 264 with one pointer per prime; with a heap, avoid pushing the same number many times

LeetCode 240 note: start at the top-right corner, moving left when larger and down when smaller, eliminating a row or column each step

LeetCode 229 note: at most two values can exceed n / 3; Boyer-Moore keeps two candidates and a second pass verifies their counts

LeetCode 216 note: use only 1 to 9, each at most once, choosing exactly k numbers that sum to n

LeetCode 204 note: the sieve of Eratosthenes, crossing out multiples of each prime p starting at p², with the outer loop only up to √n