Continuous Subarray Sum

LC 523 — find a contiguous subarray (length ≥ 2) whose sum is a multiple of `k`. Asked as the second E5 phone-screen problem. Requirements Input: integer array nums, integer k. Return True iff there is a contiguous subarray of length ≥ 2 whose sum is a non-zero multiple of k (or any multiple if k == 0). Notes Prefix-sum modulo k trick: store the first index where each prefix_sum % k value was seen. If the same remainder appears at indices i < j with j - i ≥ 2, the subarray (i, j] is a multiple of k. Edge cases: zeros in the array (two consecutive zeros are always a valid subarray), k == 0 (degenerates to finding two consecutive zeros), negative numbers (% semantics differ across languages — verify your language's behavior). Preparation Memorize the seed entry: seen = {0: -1} so a subarray starting at index 0 still satisfies the j - i ≥ 2 constraint. Be ready to defend the language-specific % behavior — interviewers often probe this for C++ / Java candidates.