523. Continuous Subarray Sum

class Solution {public:  bool checkSubarraySum(vector<int>& nums, int k) {        int n = nums.size(), sum = 0, pre = 0;        unordered_set<int> modk;        for (int i = 0; i < n; ++i) {            sum += nums[i];            int mod = k == 0 ? sum : sum % k;            if (modk.count(mod)) return true;            modk.insert(pre);            pre = mod;        }        return false;    }};