LeetCode

LeetCode 1304 note: place 1..n/2 and their negatives so each pair cancels, adding 0 when n is odd

LeetCode 3622 note: compute the digit sum and product and test n % (sum + product) == 0, keeping the modulo the right way around

LeetCode 3619 note: flood fill each island and sum its values; with up to 10^5 cells, iterate to avoid deep recursion

LeetCode 3618 note: primality is about the index; a sieve of Eratosthenes marks all of 0..n-1 at once, faster than trial division per index

LeetCode 3423 note: circular adjacency includes the last and first elements; starting from i = 0 with nums[i - 1] covers it

LeetCode 2402 note: one heap for free rooms and one for busy rooms; a delayed meeting keeps its original length

LeetCode 958 note: completeness is a global property, so check with BFS whether any node follows the first gap

LeetCode 927 note: every part has the same number of 1s, so count them; the third part always runs to the end of the array, fixing what each part must match, then check the parts do not overlap

LeetCode 1008 note: splitting the array is O(n^2); recursing with an upper bound is O(n)

LeetCode 2391 note: pickup time is the total garbage count; each truck only drives to the last house with its type, using a prefix sum of travel

LeetCode 1410 note: scan left to right, trying all six entities at each position and skipping past a match; chained replace does not work

LeetCode 1290 note: walk head to tail, shifting the running value left by one and adding the new bit