solves h index in java

Author: anishLearnsToCode

README.md

@@ -223,7 +223,7 @@
 | 268  | [Missing Number](https://leetcode.com/problems/missing-number)                                                                                            |                                                               [![Java](assets/java.png)](src/MissingNumber.java) [![Python](assets/python.png)](python/missing_number.py)                                                                |                                                                                                                                   |
 | 270  | 🔒 [Closest Binary Search Tree Value](https://leetcode.com/problems/closest-binary-search-tree-value)                                                     |                                                                                                                                                                                                                                          |                                                                                                                                   |
 | 271  | 🔒 [Encode and Decode Strings](https://leetcode.com/problems/encode-and-decode-strings)                                                                   |                                                                                                                                                                                                                                          |                                                                                                                                   |
-| 274  | [H-Index](https://leetcode.com/problems/h-index)                                                                                                          |                                                                                                                                                                                                                                          |                                                                                                                                   |
+| 274  | [H-Index](https://leetcode.com/problems/h-index)                                                                                                          |                                                                                                                 [![Java](assets/java.png)](src/HIndex.java)                                                                                                                         |                                                                                                                                   |
 | 275  | [H-Index II](https://leetcode.com/problems/h-index-ii)                                                                                                    |                                                                                                                                                                                                                                          |                                                                                                                                   |
 | 276  | 🔒 [Paint Fence](https://leetcode.com/problems/paint-fence)                                                                                               |                                                                                                                                                                                                                                          |                                                                                                                                   |
 | 277  | 🔒 [Find The Celebrity](https://leetcode.com/problems/find-the-celebrity)                                                                                 |                                                                                                                                                                                                                                          |                                                                                                                                   |

src/HIndex.java

@@ -0,0 +1,34 @@
+// https://leetcode.com/problems/h-index
+// T: O(N)
+// S: O(N)
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class HIndex {
+    public int hIndex(int[] citations) {
+        final Map<Integer, Integer> citationFrequencies = getFrequencies(citations);
+        final int maxCitation = max(citations);
+        for (int hIndex = maxCitation, papers = 0 ; hIndex >= 0 ; hIndex--) {
+            papers += citationFrequencies.getOrDefault(hIndex, 0);
+            if (papers >= hIndex) return hIndex;
+        }
+        return 0;
+    }
+
+    private int max(int[] array) {
+        int result = 0;
+        for (int element : array) {
+            result = Math.max(result, element);
+        }
+        return result;
+    }
+
+    private Map<Integer, Integer> getFrequencies(int[] array) {
+        final Map<Integer, Integer> frequencies = new HashMap<>();
+        for (int element : array) {
+            frequencies.put(element, frequencies.getOrDefault(element, 0) + 1);
+        }
+        return frequencies;
+    }
+}