solves number of equivalent domino pairs

Author: anishLearnsToCode

README.md

@@ -300,7 +300,7 @@
 | 1118 | [Number of Days in a Month](https://leetcode.com/problems/number-of-days-in-a-month)                                                                 |                                                                                                                                                                             |                                                                                                                                   |
 | 1119 | [Remove Vowels From String](https://leetcode.com/problems/remove-vowels-from-a-string)                                                               |                                                                                                                                                                             |                                                                                                                                   |
 | 1122 | [Relative Sort Array](https://leetcode.com/problems/relative-sort-array)                                                                             |                                                                                                                                                                             |                                                                                                                                   |
-| 1128 | [Number of Equivalent Domino Pairs](https://leetcode.com/problems/number-of-equivalent-domino-pairs)                                                 |                                                                                                                                                                             |                                                                                                                                   |
+| 1128 | [Number of Equivalent Domino Pairs](https://leetcode.com/problems/number-of-equivalent-domino-pairs)                                                 |                                                     [![Java](assets/java.png)](src/NumberOfEquivalentDominoPairs.java)                                                      |                                                                                                                                   |
 | 1133 | [Largest Unique Number](https://leetcode.com/problems/largest-unique-number)                                                                         |                                                                                                                                                                             |                                                                                                                                   |
 | 1134 | [Armstrong Number](https://leetcode.com/problems/armstrong-number)                                                                                   |                                                                                                                                                                             |                                                                                                                                   |
 | 1137 | [Nth Tribonacci Number]()                                                                                                                            |                                                                                                                                                                             |                                                                                                                                   |

src/NumberOfEquivalentDominoPairs.java

@@ -0,0 +1,37 @@
+import java.util.HashMap;
+import java.util.Map;
+
+public class NumberOfEquivalentDominoPairs {
+    private static class Domino {
+        private final int number;
+
+        Domino(int[] array) {
+            number = Math.min(array[0], array[1]) * 10 + Math.max(array[0], array[1]);
+        }
+
+        @Override
+        public boolean equals(Object o) {
+            Domino domino = (Domino) o;
+            return number == domino.number;
+        }
+
+        @Override
+        public int hashCode() {
+            return number;
+        }
+    }
+
+    public int numEquivDominoPairs(int[][] dominoes) {
+        final Map<Domino, Integer> dominoesFrequencies = new HashMap<>();
+        Domino d;
+        for (int[] domino : dominoes) {
+            d = new Domino(domino);
+            dominoesFrequencies.put(d, dominoesFrequencies.getOrDefault(d, 0) + 1);
+        }
+        int equivalentPairs = 0;
+        for (int frequency : dominoesFrequencies.values()) {
+            equivalentPairs += (frequency * (frequency - 1)) / 2;
+        }
+        return equivalentPairs;
+    }
+}