## The challenge#

Write a function that will return the count of distinct case-insensitive alphabetic characters and numeric digits that occur more than once in the input string. The input string can be assumed to contain only alphabets (both uppercase and lowercase) and numeric digits.

### Example#

“abcde” -> 0 `# no characters repeats more than once`
“aabbcde” -> 2 `# 'a' and 'b'`
“aabBcde” -> 2 `# 'a' occurs twice and 'b' twice (`b` and `B`)`
“indivisibility” -> 1 `# 'i' occurs six times`
“Indivisibilities” -> 2 `# 'i' occurs seven times and 's' occurs twice`
“aA11” -> 2 `# 'a' and '1'`
“ABBA” -> 2 `# 'A' and 'B' each occur twice`

## The solution in Java code#

Option 1 (using a `HashMap`):

``````import java.util.*;

public class CountingDuplicates {
public static int duplicateCount(String text) {
Map<String, Integer> map = new HashMap<>();
int ans = 0;

for (int i=0; i<text.length(); i++)
map.merge(text.toLowerCase().charAt(i)+"", 1, Integer::sum);

for (Integer value : map.values())
if (value>1) ans++;

return ans;
}
}
``````

Option 2 (using a `substring`):

``````public class CountingDuplicates {
public static int duplicateCount(String text) {
int ans = 0;
text = text.toLowerCase();
while (text.length() > 0) {
String firstLetter = text.substring(0,1);
text = text.substring(1);
if (text.contains(firstLetter)) ans ++;
text = text.replace(firstLetter, "");
}
return ans;
}
}
``````

Option 3 (using `Streams`):

``````import java.util.Map;
import static java.util.function.Function.identity;
import static java.util.stream.Collectors.counting;
import static java.util.stream.Collectors.groupingBy;

class CountingDuplicates {
private static Map<Character, Long> charFrequenciesMap(final String text) {
return text.codePoints()
.map(Character::toLowerCase)
.mapToObj(c -> (char) c)
.collect(groupingBy(identity(), counting()));
}

static int duplicateCount(final String text) {
return (int) charFrequenciesMap(text).values().stream()
.filter(i -> i > 1)
.count();
}
}
``````

## Test cases to validate our solution#

``````import org.junit.Test;
import static org.junit.Assert.assertEquals;
import org.junit.runners.JUnit4;

public class SolutionTest {
@Test
public void abcdeReturnsZero() {
assertEquals(0, CountingDuplicates.duplicateCount("abcde"));
}

@Test
public void abcdeaReturnsOne() {
assertEquals(1, CountingDuplicates.duplicateCount("abcdea"));
}

@Test
public void indivisibilityReturnsOne() {
assertEquals(1, CountingDuplicates.duplicateCount("indivisibility"));
}

@Test
public void reallyLongStringContainingDuplicatesReturnsThree() {
String testThousandA = new String(new char[1000]).replace('\0', 'a');
String testHundredB = new String(new char[100]).replace('\0', 'b');
String testTenC = new String(new char[10]).replace('\0', 'c');
String test1CapitalA = new String(new char[1]).replace('\0', 'A');
String test1d = new String(new char[1]).replace('\0', 'd');
String test = test1d + test1CapitalA + testTenC +
testHundredB + testThousandA;

assertEquals(3, CountingDuplicates.duplicateCount(test));
}

}
``````