BAEL-2337: Find Substring which are palindromes (#5766)

* BAEL-2337: Added test cases to find substring which are palindromes

* BAEL-2337: Changed the names of the variables

* BAEL-2237: Formatted code using formatter.xml

* BAEL-2237: Renamed method and added variety of test cases

* BAEL-2337: Renamed unit test cases name according to BBD style naming convention

java-strings/src/main/java/com/baeldung/string/SubstringPalindrome.java

@@ -0,0 +1,90 @@
+package com.baeldung.string;
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class SubstringPalindrome {
+
+    public Set<String> findAllPalindromesUsingCenter(String input) {
+        final Set<String> palindromes = new HashSet<>();
+        if (input == null || input.isEmpty()) {
+            return palindromes;
+        }
+        if (input.length() == 1) {
+            palindromes.add(input);
+            return palindromes;
+        }
+        for (int i = 0; i < input.length(); i++) {
+            palindromes.addAll(findPalindromes(input, i, i + 1));
+            palindromes.addAll(findPalindromes(input, i, i));
+        }
+        return palindromes;
+    }
+
+    private Set<String> findPalindromes(String input, int low, int high) {
+        Set<String> result = new HashSet<>();
+        while (low >= 0 && high < input.length() && input.charAt(low) == input.charAt(high)) {
+            result.add(input.substring(low, high + 1));
+            low--;
+            high++;
+        }
+        return result;
+    }
+
+    public Set<String> findAllPalindromesUsingBruteForceApproach(String input) {
+        Set<String> palindromes = new HashSet<>();
+        if (input == null || input.isEmpty()) {
+            return palindromes;
+        }
+        if (input.length() == 1) {
+            palindromes.add(input);
+            return palindromes;
+        }
+        for (int i = 0; i < input.length(); i++) {
+            for (int j = i + 1; j <= input.length(); j++)
+                if (isPalindrome(input.substring(i, j))) {
+                    palindromes.add(input.substring(i, j));
+                }
+        }
+        return palindromes;
+    }
+
+    private boolean isPalindrome(String input) {
+        StringBuilder plain = new StringBuilder(input);
+        StringBuilder reverse = plain.reverse();
+        return (reverse.toString()).equals(input);
+    }
+
+    public Set<String> findAllPalindromesUsingManachersAlgorithm(String input) {
+        Set<String> palindromes = new HashSet<>();
+        String formattedInput = "@" + input + "#";
+        char inputCharArr[] = formattedInput.toCharArray();
+        int max;
+        int radius[][] = new int[2][input.length() + 1];
+        for (int j = 0; j <= 1; j++) {
+            radius[j][0] = max = 0;
+            int i = 1;
+            while (i <= input.length()) {
+                palindromes.add(Character.toString(inputCharArr[i]));
+                while (inputCharArr[i - max - 1] == inputCharArr[i + j + max])
+                    max++;
+                radius[j][i] = max;
+                int k = 1;
+                while ((radius[j][i - k] != max - k) && (k < max)) {
+                    radius[j][i + k] = Math.min(radius[j][i - k], max - k);
+                    k++;
+                }
+                max = Math.max(max - k, 0);
+                i += k;
+            }
+        }
+        for (int i = 1; i <= input.length(); i++) {
+            for (int j = 0; j <= 1; j++) {
+                for (max = radius[j][i]; max > 0; max--) {
+                    palindromes.add(input.substring(i - max - 1, max + j + i - 1));
+                }
+            }
+        }
+        return palindromes;
+    }
+}

java-strings/src/test/java/com/baeldung/string/SubstringPalindromeUnitTest.java

@@ -0,0 +1,83 @@
+package com.baeldung.string;
+
+import static org.junit.Assert.assertEquals;
+import java.util.HashSet;
+import java.util.Set;
+import org.junit.Test;
+
+public class SubstringPalindromeUnitTest {
+
+    private static final String INPUT_BUBBLE = "bubble";
+    private static final String INPUT_CIVIC = "civic";
+    private static final String INPUT_INDEED = "indeed";
+    private static final String INPUT_ABABAC = "ababac";
+
+    Set<String> EXPECTED_PALINDROME_BUBBLE = new HashSet<String>() {
+        {
+            add("b");
+            add("u");
+            add("l");
+            add("e");
+            add("bb");
+            add("bub");
+        }
+    };
+
+    Set<String> EXPECTED_PALINDROME_CIVIC = new HashSet<String>() {
+        {
+            add("civic");
+            add("ivi");
+            add("i");
+            add("c");
+            add("v");
+        }
+    };
+
+    Set<String> EXPECTED_PALINDROME_INDEED = new HashSet<String>() {
+        {
+            add("i");
+            add("n");
+            add("d");
+            add("e");
+            add("ee");
+            add("deed");
+        }
+    };
+
+    Set<String> EXPECTED_PALINDROME_ABABAC = new HashSet<String>() {
+        {
+            add("a");
+            add("b");
+            add("c");
+            add("aba");
+            add("bab");
+            add("ababa");
+        }
+    };
+
+    private SubstringPalindrome palindrome = new SubstringPalindrome();
+
+    @Test
+    public void whenUsingManachersAlgorithm_thenFindsAllPalindromes() {
+        assertEquals(EXPECTED_PALINDROME_BUBBLE, palindrome.findAllPalindromesUsingManachersAlgorithm(INPUT_BUBBLE));
+        assertEquals(EXPECTED_PALINDROME_INDEED, palindrome.findAllPalindromesUsingManachersAlgorithm(INPUT_INDEED));
+        assertEquals(EXPECTED_PALINDROME_CIVIC, palindrome.findAllPalindromesUsingManachersAlgorithm(INPUT_CIVIC));
+        assertEquals(EXPECTED_PALINDROME_ABABAC, palindrome.findAllPalindromesUsingManachersAlgorithm(INPUT_ABABAC));
+    }
+
+    @Test
+    public void whenUsingCenterApproach_thenFindsAllPalindromes() {
+        assertEquals(EXPECTED_PALINDROME_BUBBLE, palindrome.findAllPalindromesUsingCenter(INPUT_BUBBLE));
+        assertEquals(EXPECTED_PALINDROME_INDEED, palindrome.findAllPalindromesUsingCenter(INPUT_INDEED));
+        assertEquals(EXPECTED_PALINDROME_CIVIC, palindrome.findAllPalindromesUsingCenter(INPUT_CIVIC));
+        assertEquals(EXPECTED_PALINDROME_ABABAC, palindrome.findAllPalindromesUsingCenter(INPUT_ABABAC));
+    }
+
+    @Test
+    public void whenUsingBruteForceApproach_thenFindsAllPalindromes() {
+        assertEquals(EXPECTED_PALINDROME_BUBBLE, palindrome.findAllPalindromesUsingBruteForceApproach(INPUT_BUBBLE));
+        assertEquals(EXPECTED_PALINDROME_INDEED, palindrome.findAllPalindromesUsingBruteForceApproach(INPUT_INDEED));
+        assertEquals(EXPECTED_PALINDROME_CIVIC, palindrome.findAllPalindromesUsingBruteForceApproach(INPUT_CIVIC));
+        assertEquals(EXPECTED_PALINDROME_ABABAC, palindrome.findAllPalindromesUsingBruteForceApproach(INPUT_ABABAC));
+    }
+}