solves #2784: Check if Array is Good in java

Author: anishLearnsToCode

README.txt

@@ -839,7 +839,7 @@
 | 2765 | [Longest Alternating Subarray](https://leetcode.com/problems/longest-alternating-subarray)                                                                             |                                                                                   [![Java](assets/java.png)](src/LongestAlternatingSubarray.java)                                                                                                                                               |                                                                                                                                   |
 | 2769 | [Find the Maximum Achievable Number](https://leetcode.com/problems/find-the-maximum-achievable-number)                                                                 |                                                                                   [![Java](assets/java.png)](src/FindTheMaximumAchievableNumber.java)                                                                                                                                               |                                                                                                                                   |
 | 2778 | [Sum of Squares of Special Elements](https://leetcode.com/problems/sum-of-squares-of-special-elements)                                                                 |                                                                                   [![Java](assets/java.png)](src/SumOfSquaresOfSpecialElements.java)                                                                                                                                               |                                                                                                                                   |
-| 2784 | [Check if Array is Good](https://leetcode.com/problems/check-if-array-is-good)                                                                                         |                                                                                                                                                                                                                                  |                                                                                                                                   |
+| 2784 | [Check if Array is Good](https://leetcode.com/problems/check-if-array-is-good)                                                                                         |                                                                                   [![Java](assets/java.png)](src/CheckIfArrayIsGood.java)                                                                                                                                               |                                                                                                                                   |
 | 2788 | [Split Strings by Separator](https://leetcode.com/problems/split-strings-by-separator)                                                                                 |                                                                                                                                                                                                                                  |                                                                                                                                   |
 | 2798 | [Number of Employees Who Met the Target](https://leetcode.com/problems/number-of-employees-who-met-the-target)                                                         |                                                                                                                                                                                                                                  |                                                                                                                                   |
 | 2806 | [Account Balance After Rounded Purchase](https://leetcode.com/problems/account-balance-after-rounded-purchase)                                                         |                                                                                                                                                                                                                                  |                                                                                                                                   |

src/CheckIfArrayIsGood.java

@@ -0,0 +1,26 @@
+// https://leetcode.com/problems/check-if-array-is-good
+// T: O(N)
+// S: O(N)
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class CheckIfArrayIsGood {
+    public boolean isGood(int[] array) {
+        final Map<Integer, Integer> frequencies = getFrequencies(array);
+        for (int i = 1 ; i < array.length - 1 ; i++) {
+            if (frequencies.getOrDefault(i, 0) != 1) {
+                return false;
+            }
+        }
+        return frequencies.getOrDefault(array.length - 1, 0) == 2;
+    }
+
+    private Map<Integer, Integer> getFrequencies(int[] array) {
+        final Map<Integer, Integer> result = new HashMap<>();
+        for (int element : array) {
+            result.put(element, result.getOrDefault(element, 0) + 1);
+        }
+        return result;
+    }
+}