solves number of good pairs

Author: anishLearnsToCode

README.md

@@ -382,7 +382,7 @@
 | 1496 | [Path Crossing](https://leetcode.com/problems/path-crossing)                                                                                         |                                                                  [![Java](assets/java.png)](src/PathCrossing.java)                                                                   |                                                                                                                                   |
 | 1502 | [Can Make Arithmetic Progression From Sequence](https://leetcode.com/problems/can-make-arithmetic-progression-from-sequence)                         |                                                    [![Java](assets/java.png)](src/CanMakeArithmeticProgressionFromSequence.java)                                                     |                                                                                                                                   |
 | 1507 | [Reformat Date](https://leetcode.com/problems/reformat-date)                                                                                         |                                                                  [![Java](assets/java.png)](src/ReformatDate.java)                                                                   |                                                                                                                                   |
-| 1512 | [Number of Good Pairs](https://leetcode.com/problems/number-of-good-pairs)                                                                           |                                                                                                                                                                                      |                                                                                                                                   |
+| 1512 | [Number of Good Pairs](https://leetcode.com/problems/number-of-good-pairs)                                                                           |                                                                [![Java](assets/java.png)](src/NumberOfGoodPairs.java)                                                                |                                                                                                                                   |
 | 1518 | [Water Bottles](https://leetcode.com/problems/water-bottles)                                                                                         |                                                                                                                                                                                      |                                                                                                                                   |
 | 1523 | [Count Odd Numbers In Interval Range](https://leetcode.com/problems/count-odd-numbers-in-an-interval-range)                                          |                                                                                                                                                                                      |                                                                                                                                   |
 | 1528 | [Shuffle Strings](https://leetcode.com/problems/shuffle-string)                                                                                      |                                                                                                                                                                                      |                                                                                                                                   |

src/NumberOfGoodPairs.java

@@ -0,0 +1,29 @@
+// https://leetcode.com/problems/number-of-good-pairs
+// T: O(n)
+// S: O(n)
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class NumberOfGoodPairs {
+    public int numIdenticalPairs(int[] nums) {
+        Map<Integer, Integer> frequencies = getFrequencies(nums);
+        int goodPairs = 0;
+        for (int frequency : frequencies.values()) {
+            goodPairs += nC2(frequency);
+        }
+        return goodPairs;
+    }
+
+    private int nC2(int x) {
+        return (x * (x - 1)) / 2;
+    }
+
+    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;
+    }
+}