One-Hundred-Sixty-Five Commit: Add Frequency Sort problem to Top K Elements section

Author: AAdewunmi

src/Top_K_Elements/Problem_6_Frequency_Sort.java

@@ -0,0 +1,32 @@
+package Top_K_Elements;
+
+// Problem Statement: Frequency Sort (medium)
+// LeetCode Question: 451. Sort Characters By Frequency
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.PriorityQueue;
+
+public class Problem_6_Frequency_Sort {
+    public String sortCharacterByFrequency(String str) {
+
+        Map<Character, Integer> characterFrequencyMap = new HashMap<>();
+        for (char chr : str.toCharArray()) {
+            characterFrequencyMap.put(chr, characterFrequencyMap.getOrDefault(chr, 0) + 1);
+        }
+
+        PriorityQueue<Map.Entry<Character, Integer>> maxHeap =
+                new PriorityQueue<Map.Entry<Character, Integer>>(
+                        (e1, e2) -> e2.getValue() - e1.getValue());
+
+        maxHeap.addAll(characterFrequencyMap.entrySet());
+
+        StringBuilder sortedString = new StringBuilder(str.length());
+        while (!maxHeap.isEmpty()) {
+            Map.Entry<Character, Integer> entry = maxHeap.poll();
+            for (int i = 0; i < entry.getValue(); i++)
+                sortedString.append(entry.getKey());
+        }
+        return sortedString.toString();
+    }
+}