Math

LeetCode 507 note: divisors come in pairs, so enumerate up to √n and add both d and n / d for each divisor found

LeetCode 504 note: repeatedly take % 7 and // 7 of the absolute value, then add the sign and reverse; handle 0 separately

LeetCode 492 note: search the width downward from √area; the first divisor of area gives the most square-like rectangle

LeetCode 441 note: find the largest k with k(k + 1) / 2 <= n; when moving right write l = mid + 1 or the search gets stuck

LeetCode 415 note: add digit by digit from the ends with two pointers, tracking the carry, and append any leftover carry after the loop

LeetCode 412 note: check multiples of 15 first, then 5 and 3; multiples need %, not //

LeetCode 405 note: add 2^32 to a negative number to get its two’s complement, then take 4 bits at a time (% 16); handle 0 separately

LeetCode 367 note: binary search [1, num] for mid² == num without using sqrt

LeetCode 342 note: confirm a power of two, then that the single 1 sits at an even position: n & 0x55555555 != 0, or n % 3 == 1

LeetCode 326 note: divide by 3 while possible and check for 1; without loops, n is a power of three exactly when it divides 3^19

LeetCode 292 note: whoever faces a multiple of 4 stones loses, so the first player wins when n % 4 != 0

LeetCode 268 note: the sum of 0..n minus the array’s sum is the missing number, which may be 0