Math

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

LeetCode 3550 note: check indices in increasing order, computing each digit sum; the first match is the answer

LeetCode 1979 note: find the minimum and maximum, then take their GCD with Euclid’s algorithm

LeetCode 952 note: sharing a factor ⇔ sharing a prime factor; factor each number and union indices with the same prime factor using Union-Find, with no pairwise GCDs

LeetCode 3536 note: multiply the largest and second-largest digits; when a new maximum appears, the old maximum must move to second place

LeetCode 3518 note: a palindrome is determined by its left half, so this is the k-th permutation of a multiset; try characters position by position and use permutation counts, updated in O(1), to find k’s branch

LeetCode 3492 note: at most n² cells and at most maxWeight // w by weight, so take the smaller, using integer division

LeetCode 1814 note: rearrange the condition into nums[i] - rev(nums[i]) == nums[j] - rev(nums[j]), count pairs per value, and remember the modulus

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