523. Continuous Subarray Sum

Given a list of non-negative numbers and a target integer k, write a function to check if the array has a continuous subarray of size at least 2 that sums up to a multiple of k, that is, sums up to n*k where n is also an integer.

 

Example 1:

Input: [23, 2, 4, 6, 7],  k=6
Output: True
Explanation: Because [2, 4] is a continuous subarray of size 2 and sums up to 6.

Example 2:

Input: [23, 2, 6, 4, 7],  k=6
Output: True
Explanation: Because [23, 2, 6, 4, 7] is an continuous subarray of size 5 and sums up to 42.

class Solution {
    public boolean checkSubarraySum(int[] nums, int k) {
        Map<Integer, Integer> map = new HashMap<>();
        map.put(0, -1);
        
        int runningSum = 0;
        for (int i = 0; i < nums.length; i++) {
            runningSum += nums[i];
            if (k != 0) {
                runningSum %= k;
            }
            Integer prev = map.get(runningSum);
            if (prev != null) {
                if (i - prev > 1) {
                    return true;
                }
            } else {
                map.put(runningSum, i);
            }
        }
        return false;
    }
}

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