523_Continuous_Subarray_Sum.cpp

/*
523. Continuous Subarray Sum
Given an integer array nums and an integer k, return true if nums has a good subarray or false otherwise.
A good subarray is a subarray where:
its length is at least two, and
the sum of the elements of the subarray is a multiple of k.
Note that:
A subarray is a contiguous part of the array.
An integer x is a multiple of k if there exists an integer n such that x = n * k. 0 is always a multiple of k.
Example 1
Input: nums = [23,2,4,6,7], k = 6
Output: true
Explanation: [2, 4] is a continuous subarray of size 2 whose elements sum up to 6.
Example 2:
Input: nums = [23,2,6,4,7], k = 6
Output: true
Explanation: [23, 2, 6, 4, 7] is an continuous subarray of size 5 whose elements sum up to 42.
42 is a multiple of 6 because 42 = 7 * 6 and 7 is an integer.
Example 3:
Input: nums = [23,2,6,4,7], k = 13
Output: false
Constraints:
1 <= nums.length <= 105
0 <= nums[i] <= 109
0 <= sum(nums[i]) <= 231 - 1
1 <= k <= 231 - 1
*/
class Solution {
public:
    bool checkSubarraySum(vector<int>& nums, int k) {
        unordered_map<int, int> map;
        map[0] = -1; // To handle the case where the sub-array starts from index 0
        int sum = 0;

        for (int i = 0; i < nums.size(); i++) {
            sum += nums[i];
            int rem = sum % k;

            if (map.find(rem) != map.end()) {
                if (i - map[rem] > 1) {
                    return true;
                }
            } else {
                map[rem] = i;
            }
        }

        return false;
    }
};