add a solution for 1636

Author: fishercoder1534

Diff for: src/main/java/com/fishercoder/solutions/secondthousand/_1636.java

@@ -1,6 +1,7 @@
 package com.fishercoder.solutions.secondthousand;
 
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
@@ -39,4 +40,30 @@ public int[] frequencySort(int[] nums) {
             return result;
         }
     }
+
+    public static class Solution2 {
+
+        public int[] frequencySort(int[] nums) {
+            Map<Integer, Integer> map = new HashMap<>();
+            for (int num : nums) {
+                map.put(num, map.getOrDefault(num, 0) + 1);
+            }
+            int[][] pairs = new int[map.size()][2];
+            int i = 0;
+            for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
+                pairs[i][0] = entry.getKey();
+                pairs[i++][1] = entry.getValue();
+            }
+            Arrays.sort(pairs, (a, b) -> a[1] != b[1] ? a[1] - b[1] : b[0] - a[0]);
+            int[] result = new int[nums.length];
+            i = 0;
+            for (int[] pair : pairs) {
+                int count = pair[1];
+                while (count-- > 0) {
+                    result[i++] = pair[0];
+                }
+            }
+            return result;
+        }
+    }
 }

Diff for: src/test/java/com/fishercoder/secondthousand/_1636Test.java

@@ -1,23 +1,26 @@
 package com.fishercoder.secondthousand;
 
 import com.fishercoder.solutions.secondthousand._1636;
-import org.junit.BeforeClass;
-import org.junit.Test;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
 
-import static org.junit.Assert.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 
 public class _1636Test {
     private static _1636.Solution1 solution1;
+    private static _1636.Solution2 solution2;
     private static int[] nums;
 
-    @BeforeClass
-    public static void setup() {
+    @BeforeEach
+    public void setup() {
         solution1 = new _1636.Solution1();
+        solution2 = new _1636.Solution2();
     }
 
     @Test
     public void test1() {
         nums = new int[]{1, 1, 2, 2, 2, 3};
+        assertArrayEquals(new int[]{3, 1, 1, 2, 2, 2}, solution2.frequencySort(nums));
         assertArrayEquals(new int[]{3, 1, 1, 2, 2, 2}, solution1.frequencySort(nums));
     }