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solves #2748: Number of Beautiful Pairs in java

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`834`	`834`	`\| 2733 \| [Neither Minimum nor Maximum](https://leetcode.com/problems/neither-minimum-nor-maximum) \| [![Java](assets/java.png)](src/NeitherMinimumNorMaximum.java) \| \|`
`835`	`835`	`\| 2739 \| [Total Distance Traveled](https://leetcode.com/problems/total-distance-traveled) \| [![Java](assets/java.png)](src/TotalDistanceTraveled.java) \| \|`
`836`	`836`	`\| 2744 \| [Find Maximum Number of String Pairs](https://leetcode.com/problems/find-maximum-number-of-string-pairs) \| [![Java](assets/java.png)](src/FindMaximumNumberOfStringPairs.java) \| \|`
`837`		`-\| 2748 \| [Number of Beautiful Pairs](https://leetcode.com/problems/number-of-beautiful-pairs) \| \| \|`
	`837`	`+\| 2748 \| [Number of Beautiful Pairs](https://leetcode.com/problems/number-of-beautiful-pairs) \| [![Java](assets/java.png)](src/)NumberOfBeautifulPairs.java) \| \|`
`838`	`838`	`\| 2760 \| [Longest Even Odd Subarray With Threshold](https://leetcode.com/problems/longest-even-odd-subarray-with-threshold) \| \| \|`
`839`	`839`	`\| 2765 \| [Longest Alternating Subarray](https://leetcode.com/problems/longest-alternating-subarray) \| \| \|`
`840`	`840`	`\| 2769 \| [Find the Maximum Achievable Number](https://leetcode.com/problems/find-the-maximum-achievable-number) \| \| \|`

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	`1`	`+// https://leetcode.com/problems/number-of-beautiful-pairs/description`
	`2`	`+// N: length of number array`
	`3`	`+// m: size of average number`
	`4`	`+// T: O(N * log(m))`
	`5`	`+// S: O(N)`
	`6`	`+`
	`7`	`+import java.util.HashMap;`
	`8`	`+import java.util.Map;`
	`9`	`+`
	`10`	`+public class NumberOfBeautifulPairs {`
	`11`	`+ public int countBeautifulPairs(int[] array) {`
	`12`	`+ final Map<Integer, Integer> firstDigitFrequencies = new HashMap<>();`
	`13`	`+ int pairs = 0;`
	`14`	`+ for (int element : array) {`
	`15`	`+ final int firstDigit = getFirstDigit(element);`
	`16`	`+ final int lastDigit = getLastDigit(element);`
	`17`	`+ for (Map.Entry<Integer, Integer> entry : firstDigitFrequencies.entrySet()) {`
	`18`	`+ if (gcd(lastDigit, entry.getKey()) == 1) {`
	`19`	`+ pairs += entry.getValue();`
	`20`	`+ }`
	`21`	`+ }`
	`22`	`+ firstDigitFrequencies.put(firstDigit, firstDigitFrequencies.getOrDefault(firstDigit, 0) + 1);`
	`23`	`+ }`
	`24`	`+ return pairs;`
	`25`	`+ }`
	`26`	`+`
	`27`	`+ private int getFirstDigit(int number) {`
	`28`	`+ return (number + "").charAt(0) - '0';`
	`29`	`+ }`
	`30`	`+`
	`31`	`+ private int getLastDigit(int number) {`
	`32`	`+ return number % 10;`
	`33`	`+ }`
	`34`	`+`
	`35`	`+ // T: O(log(max(a, b)))`
	`36`	`+ // S: O(log(max(a, b)))`
	`37`	`+ private int gcd(int a, int b) {`
	`38`	`+ if (b == 0) return a;`
	`39`	`+ return gcd(b, a % b);`
	`40`	`+ }`
	`41`	`+}`

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