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added isogram utility class and unit tests
Signed-off-by: JeevaRamanathan <jeevaramanathan.m@infosys.com>
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‎src/main/java/com/thealgorithms/strings/Isogram.java‎

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package com.thealgorithms.strings;
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import java.util.HashSet;
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import java.util.Set;
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/**
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* An isogram (also called heterogram or nonpattern word) is a word in which no
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* letter of the word occurs more than once. Each character appears exactly
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* once.
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*
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* For example, the word "uncopyrightable" is the longest common English isogram
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* with 15 unique letters. Other examples include "dermatoglyphics" (15
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* letters),
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* "background" (10 letters), "python" (6 letters), and "keyboard" (8 letters).
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* But words like "hello" and "programming" are not isograms because some
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* letters
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* appear multiple times ('l' appears twice in "hello", while 'r', 'm', 'g'
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* repeat
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* in "programming").
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*
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* Isograms are particularly valuable in creating substitution ciphers and are
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* studied in recreational linguistics. A perfect pangram, which uses all 26
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* letters
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* of the alphabet exactly once, is a special type of isogram.
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*
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* Reference from https://en.wikipedia.org/wiki/Heterogram_(literature)#Isograms
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*/
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public class Isogram {
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/**
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* Checks if a string is an isogram using boolean array approach.
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*
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* Time Complexity: O(n)
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* Space Complexity: O(1)
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*
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* @param str the input string
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* @return true if the string is an isogram, false otherwise
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* @throws IllegalArgumentException if the string contains non-alphabetic
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* characters
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*/
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public static boolean isIsogramByArray(String str) {
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if (str == null || str.isEmpty()) {
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return true;
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}
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String lowerStr = str.toLowerCase();
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boolean[] seenChars = new boolean[26];
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for (int i = 0; i < lowerStr.length(); i++) {
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char ch = lowerStr.charAt(i);
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// Check if character is a letter
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if (ch >= 'a' && ch <= 'z') {
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int index = ch - 'a';
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if (seenChars[index]) {
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return false; // Letter already seen
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}
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seenChars[index] = true;
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}
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}
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return true;
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}
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/**
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* Checks if a string is an isogram using length comparison approach.
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* Time Complexity: O(n)
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* Space Complexity: O(k) where k is the number of unique characters
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*
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* @param str the input string
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* @return true if the string is an isogram, false otherwise
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*/
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public static boolean isIsogramByLength(String str) {
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if (str == null || str.isEmpty()) {
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return true;
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}
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str = str.toLowerCase();
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Set<Character> uniqueChars = new HashSet<>();
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for (char ch : str.toCharArray()) {
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uniqueChars.add(ch);
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}
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return uniqueChars.size() == str.length();
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}
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}

‎src/test/java/com/thealgorithms/strings/IsogramTest.java‎

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package com.thealgorithms.strings;
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import static org.junit.jupiter.api.Assertions.assertEquals;
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import java.util.stream.Stream;
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import org.junit.jupiter.params.ParameterizedTest;
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import org.junit.jupiter.params.provider.MethodSource;
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import org.junit.jupiter.api.Test;
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public class IsogramTest {
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record IsogramTestCase(String input, boolean expected) {
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}
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private static Stream<IsogramTestCase> isogramArrayTestData() {
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return Stream.of(
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// Valid isograms (only checks letters)
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new IsogramTestCase("uncopyrightable", true),
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new IsogramTestCase("dermatoglyphics", true),
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new IsogramTestCase("background", true),
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new IsogramTestCase("python", true),
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new IsogramTestCase("keyboard", true),
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new IsogramTestCase("clipboard", true),
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new IsogramTestCase("flowchart", true),
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new IsogramTestCase("bankruptcy", true),
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new IsogramTestCase("computer", true),
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new IsogramTestCase("algorithms", true),
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// Not isograms - letters repeat
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new IsogramTestCase("hello", false),
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new IsogramTestCase("programming", false),
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new IsogramTestCase("java", false),
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new IsogramTestCase("coffee", false),
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new IsogramTestCase("book", false),
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new IsogramTestCase("letter", false),
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new IsogramTestCase("mississippi", false),
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new IsogramTestCase("google", false),
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// Edge cases
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new IsogramTestCase("", true),
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new IsogramTestCase("a", true),
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new IsogramTestCase("ab", true),
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new IsogramTestCase("abc", true),
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new IsogramTestCase("aa", false),
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new IsogramTestCase("abcdefghijklmnopqrstuvwxyz", true), // All 26 letters
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// Case insensitive
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new IsogramTestCase("Python", true),
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new IsogramTestCase("BACKGROUND", true),
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new IsogramTestCase("Hello", false),
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new IsogramTestCase("PROGRAMMING", false));
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}
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private static Stream<IsogramTestCase> isogramLengthTestData() {
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return Stream.of(
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// Valid isograms (checks all characters)
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new IsogramTestCase("uncopyrightable", true),
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new IsogramTestCase("dermatoglyphics", true),
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new IsogramTestCase("background", true),
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new IsogramTestCase("python", true),
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new IsogramTestCase("keyboard", true),
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new IsogramTestCase("clipboard", true),
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new IsogramTestCase("flowchart", true),
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new IsogramTestCase("bankruptcy", true),
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new IsogramTestCase("computer", true),
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new IsogramTestCase("algorithms", true),
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// Not isograms - characters repeat
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new IsogramTestCase("hello", false),
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new IsogramTestCase("programming", false),
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new IsogramTestCase("java", false),
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new IsogramTestCase("coffee", false),
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new IsogramTestCase("book", false),
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new IsogramTestCase("letter", false),
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new IsogramTestCase("mississippi", false),
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new IsogramTestCase("google", false),
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// Edge cases
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new IsogramTestCase("", true),
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new IsogramTestCase("a", true),
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new IsogramTestCase("ab", true),
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new IsogramTestCase("abc", true),
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new IsogramTestCase("aa", false),
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new IsogramTestCase("abcdefghijklmnopqrstuvwxyz", true), // All 26 letters
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// Case insensitive
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new IsogramTestCase("Python", true),
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new IsogramTestCase("BACKGROUND", true),
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new IsogramTestCase("Hello", false),
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new IsogramTestCase("PROGRAMMING", false),
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// Strings with symbols and numbers
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new IsogramTestCase("abc@def", true), // all characters unique
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new IsogramTestCase("test-case", false), // 't', 's', 'e' repeat
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new IsogramTestCase("python123", true), // all characters unique
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new IsogramTestCase("hello@123", false), // 'l' repeats
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new IsogramTestCase("abc123!@#", true), // all characters unique
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new IsogramTestCase("test123test", false), // 't', 'e', 's' repeat
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new IsogramTestCase("1234567890", true), // all digits unique
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new IsogramTestCase("12321", false), // '1' and '2' repeat
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new IsogramTestCase("!@#$%^&*()", true) // all special characters unique
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);
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}
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@ParameterizedTest
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@MethodSource("isogramArrayTestData")
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void testIsogramByArray(IsogramTestCase testCase) {
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assertEquals(testCase.expected(), Isogram.isIsogramByArray(testCase.input()));
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}
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@ParameterizedTest
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@MethodSource("isogramLengthTestData")
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void testIsogramByLength(IsogramTestCase testCase) {
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assertEquals(testCase.expected(), Isogram.isIsogramByLength(testCase.input()));
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}
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@Test
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void testNullInputByArray() {
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assertEquals(true, Isogram.isIsogramByArray(null));
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}
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@Test
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void testNullInputByLength() {
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assertEquals(true, Isogram.isIsogramByLength(null));
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}
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@Test
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void testEmptyStringByArray() {
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assertEquals(true, Isogram.isIsogramByArray(""));
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}
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@Test
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void testEmptyStringByLength() {
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assertEquals(true, Isogram.isIsogramByLength(""));
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}
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}

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