Simulation

LeetCode 657 note: returning to the origin means as many U as D and as many L as R

LeetCode 566 note: return the original if the element counts differ; otherwise element k goes to (k // c, k % c)

LeetCode 495 note: each attack contributes min(gap to the next attack, duration), and the last one contributes the full duration

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 258 note: sum the digits repeatedly until one remains; the O(1) answer is the digital root 1 + (n - 1) % 9

LeetCode 67 note: add from the ends of both strings digit by digit, with carry sum // 2 and digit sum % 2