refactor 523

Author: fishercoder1534

src/main/java/com/fishercoder/solutions/_523.java

@@ -8,17 +8,17 @@ public class _523 {
     public static class Solution1 {
         /**
          * reference: https://discuss.leetcode.com/topic/80793/java-o-n-time-o-k-space/20
-         * "The reason we use modulus is:
+         * "The reason we use modulo is:
          * (a+(n*x))%x is same as (a%x)
-         * e.g. in case of the array [23,2,6,4,7] the running sum is [23,25,31,35,42]
+         * e.g. in case of the array [23,2,6,4,7] and k = 6 the running sum is [23,25,31,35,42]
          * and the remainders are [5,1,1,5,0].
          * We got reminder 5 at index 0 and at index 3.
          * That means, in between these two indexes we must have added a number which is multiple of the k.
          * Hope this clarifies your doubt :)"
          */
         public boolean checkSubarraySum(int[] nums, int k) {
             Map<Integer, Integer> map = new HashMap<>();
-            map.put(0, -1);
+            map.put(0, -1);//this line is critical to mark the beginning of the prefix sum, so that next time, when we encounter a running sum of zero, we know that's the answer, see test case 11
             int sum = 0;
             for (int i = 0; i < nums.length; i++) {
                 sum += nums[i];
@@ -41,6 +41,9 @@ public boolean checkSubarraySum(int[] nums, int k) {
     }
 
     public static class Solution2 {
+        /**
+         * O(n^2), this will time out on LeetCode.
+         */
         public boolean checkSubarraySum(int[] nums, int k) {
             if (nums == null || nums.length == 0) {
                 return false;

src/test/java/com/fishercoder/_523Test.java

@@ -109,4 +109,13 @@ public void test10() {
         assertEquals(expected, solution2.checkSubarraySum(nums, k));
     }
 
+    @Test
+    public void test11() {
+        nums = new int[]{23, 2, 4, 6, 6};
+        expected = true;
+        k = 7;
+        assertEquals(expected, solution1.checkSubarraySum(nums, k));
+        assertEquals(expected, solution2.checkSubarraySum(nums, k));
+    }
+
 }