solves #2748: Number of Beautiful Pairs in java

Author: anishLearnsToCode

README.txt

@@ -834,7 +834,7 @@
 | 2733 | [Neither Minimum nor Maximum](https://leetcode.com/problems/neither-minimum-nor-maximum)                                                                               |                                                                                   [![Java](assets/java.png)](src/NeitherMinimumNorMaximum.java)                                                                                                                                               |                                                                                                                                   |
 | 2739 | [Total Distance Traveled](https://leetcode.com/problems/total-distance-traveled)                                                                                       |                                                                                   [![Java](assets/java.png)](src/TotalDistanceTraveled.java)                                                                                                                                               |                                                                                                                                   |
 | 2744 | [Find Maximum Number of String Pairs](https://leetcode.com/problems/find-maximum-number-of-string-pairs)                                                               |                                                                                   [![Java](assets/java.png)](src/FindMaximumNumberOfStringPairs.java)                                                                                                                                               |                                                                                                                                   |
-| 2748 | [Number of Beautiful Pairs](https://leetcode.com/problems/number-of-beautiful-pairs)                                                                                   |                                                                                                                                                                                                                                  |                                                                                                                                   |
+| 2748 | [Number of Beautiful Pairs](https://leetcode.com/problems/number-of-beautiful-pairs)                                                                                   |                                                                                   [![Java](assets/java.png)](src/)NumberOfBeautifulPairs.java)                                                                                                                                               |                                                                                                                                   |
 | 2760 | [Longest Even Odd Subarray With Threshold](https://leetcode.com/problems/longest-even-odd-subarray-with-threshold)                                                     |                                                                                                                                                                                                                                  |                                                                                                                                   |
 | 2765 | [Longest Alternating Subarray](https://leetcode.com/problems/longest-alternating-subarray)                                                                             |                                                                                                                                                                                                                                  |                                                                                                                                   |
 | 2769 | [Find the Maximum Achievable Number](https://leetcode.com/problems/find-the-maximum-achievable-number)                                                                 |                                                                                                                                                                                                                                  |                                                                                                                                   |

src/NumberOfBeautifulPairs.java

@@ -0,0 +1,41 @@
+// https://leetcode.com/problems/number-of-beautiful-pairs/description
+// N: length of number array
+// m: size of average number
+// T: O(N * log(m))
+// S: O(N)
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class NumberOfBeautifulPairs {
+    public int countBeautifulPairs(int[] array) {
+        final Map<Integer, Integer> firstDigitFrequencies = new HashMap<>();
+        int pairs = 0;
+        for (int element : array) {
+            final int firstDigit = getFirstDigit(element);
+            final int lastDigit = getLastDigit(element);
+            for (Map.Entry<Integer, Integer> entry : firstDigitFrequencies.entrySet()) {
+                if (gcd(lastDigit, entry.getKey()) == 1) {
+                    pairs += entry.getValue();
+                }
+            }
+            firstDigitFrequencies.put(firstDigit, firstDigitFrequencies.getOrDefault(firstDigit, 0) + 1);
+        }
+        return pairs;
+    }
+
+    private int getFirstDigit(int number) {
+        return (number + "").charAt(0) - '0';
+    }
+
+    private int getLastDigit(int number) {
+        return number % 10;
+    }
+
+    // T: O(log(max(a, b)))
+    // S: O(log(max(a, b)))
+    private int gcd(int a, int b) {
+        if (b == 0) return a;
+        return gcd(b, a % b);
+    }
+}