find top k frequent elements

Author: geegong

top-k-frequent-elements/Geegong.java

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+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+
+public class Geegong {
+
+
+    public int[] topKFrequent(int[] nums, int k) {
+        int[] result = new int[k];
+
+        // key : num element in nums / value : frequency of num elements
+        Map<Integer, Integer> numMap = new HashMap<>();
+
+        // key : frequency of num elements / value : HashSet<Integer> num elements
+        Map<Integer, HashSet<Integer>> frequencyMap = new HashMap<>();
+
+        // most frequent numbers
+        int maxCount = 0;
+
+        // initialize numMap
+        for (int num : nums) {
+            if (numMap.containsKey(num)) {
+                Integer alreadyCounted = numMap.get(num);
+                numMap.put(num, alreadyCounted + 1);
+            } else {
+                numMap.put(num, 1);
+            }
+        }
+
+
+        //numHashSetMap
+        for (int num : numMap.keySet()) {
+            int frequencyOfNum = numMap.get(num);
+            maxCount = Math.max(maxCount, frequencyOfNum);
+
+            if (frequencyMap.containsKey(frequencyOfNum)) {
+                HashSet<Integer> alreadySet = frequencyMap.get(frequencyOfNum);
+                alreadySet.add(num);
+
+                frequencyMap.put(frequencyOfNum, alreadySet);
+
+            } else {
+                HashSet<Integer> newHashSet = new HashSet<>();
+                newHashSet.add(num);
+
+                frequencyMap.put(frequencyOfNum, newHashSet);
+            }
+        }
+
+
+        // maxCount 부터 decreasing
+        int resultIndex=0;
+        for(int frequency=maxCount; frequency>=0; frequency--) {
+            if (resultIndex >= result.length) {
+                return result;
+            }
+
+            if (frequencyMap.containsKey(frequency)) {
+                HashSet<Integer> numElements = frequencyMap.get(frequency);
+
+                for (int numElement : numElements) {
+                    result[resultIndex] = numElement;
+                    resultIndex++;
+
+
+                    if (resultIndex >= result.length) {
+                        return result;
+                    }
+                }
+            }
+        }
+
+        return result;
+}
+