We iterate through the input array exactly once, keeping track of the running sum mod k of the elements in the process. If we find that a running sum value at index j has been previously seen before in some earlier index i in the array, then we know that the sub-array (i,j] contains a desired sum.

```
public boolean checkSubarraySum(int[] nums, int k) {
Map<Integer, Integer> map = new HashMap<Integer, Integer>(){{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;
}
```