Merge branch 'master' into add-title-case-v3

Author: Rosander0

src/main/java/com/thealgorithms/slidingwindow/CountDistinctElementsInWindow.java

@@ -0,0 +1,57 @@
+package com.thealgorithms.slidingwindow;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Counts the number of distinct elements in every window of size k.
+ *
+ * @see <a href="https://www.geeksforgeeks.org/count-distinct-elements-in-every-window-of-size-k/">Reference</a>
+ */
+public final class CountDistinctElementsInWindow {
+
+    private CountDistinctElementsInWindow() {
+    }
+
+    /**
+     * Returns an array where each element is the count of distinct
+     * elements in the corresponding window of size k.
+     *
+     * @param arr the input array
+     * @param k   the window size
+     * @return array of distinct element counts per window
+     */
+    public static int[] countDistinct(int[] arr, int k) {
+        if (arr == null || arr.length == 0 || k <= 0 || k > arr.length) {
+            throw new IllegalArgumentException("Invalid input");
+        }
+
+        int n = arr.length;
+        int[] result = new int[n - k + 1];
+        Map<Integer, Integer> freqMap = new HashMap<>();
+
+        for (int i = 0; i < k; i++) {
+            freqMap.merge(arr[i], 1, Integer::sum);
+        }
+        result[0] = freqMap.size();
+
+        for (int i = k; i < n; i++) {
+            freqMap.merge(arr[i], 1, Integer::sum);
+
+            int outgoing = arr[i - k];
+
+            Integer count = freqMap.get(outgoing);
+            if (count != null) {
+                if (count == 1) {
+                    freqMap.remove(outgoing);
+                } else {
+                    freqMap.put(outgoing, count - 1);
+                }
+            }
+
+            result[i - k + 1] = freqMap.size();
+        }
+
+        return result;
+    }
+}

src/main/java/com/thealgorithms/strings/LongestCommonSubstring.java

@@ -0,0 +1,55 @@
+package com.thealgorithms.strings;
+
+/**
+ * Longest Common Substring finds the longest string that is a
+ * contiguous substring of two input strings.
+ * Example: "abcdef" and "zcdemf" -> "cde"
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Longest_common_substring">
+ *     Wikipedia: Longest Common Substring</a>
+ *
+ * author: Vraj Prajapati @Rosander0
+ */
+public final class LongestCommonSubstring {
+
+    private LongestCommonSubstring() {
+        // Utility class
+    }
+
+    /**
+     * Finds the longest common substring of two strings.
+     *
+     * @param a First input string
+     * @param b Second input string
+     * @return The longest common substring, or empty string if none exists.
+     *     If multiple substrings share the maximum length, the first one found is returned.
+     */
+    public static String longestCommonSubstring(final String a, final String b) {
+        if (a == null || b == null || a.isEmpty() || b.isEmpty()) {
+            return "";
+        }
+
+        int[][] dp = new int[a.length() + 1][b.length() + 1];
+        int maxLength = 0;
+        int endIndex = 0;
+
+        for (int i = 1; i <= a.length(); i++) {
+            for (int j = 1; j <= b.length(); j++) {
+                if (a.charAt(i - 1) == b.charAt(j - 1)) {
+                    dp[i][j] = dp[i - 1][j - 1] + 1;
+                    if (dp[i][j] > maxLength) {
+                        maxLength = dp[i][j];
+                        endIndex = i;
+                    }
+                } else {
+                    dp[i][j] = 0;
+                }
+            }
+        }
+
+        if (maxLength == 0) {
+            return "";
+        }
+        return a.substring(endIndex - maxLength, endIndex);
+    }
+}

src/test/java/com/thealgorithms/slidingwindow/CountDistinctElementsInWindowTest.java

@@ -0,0 +1,34 @@
+package com.thealgorithms.slidingwindow;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+public class CountDistinctElementsInWindowTest {
+
+    @Test
+    public void testBasicCase() {
+        assertArrayEquals(new int[] {3, 2, 2}, CountDistinctElementsInWindow.countDistinct(new int[] {1, 2, 3, 2, 3}, 3));
+    }
+
+    @Test
+    public void testAllSame() {
+        assertArrayEquals(new int[] {1, 1, 1}, CountDistinctElementsInWindow.countDistinct(new int[] {2, 2, 2, 2}, 2));
+    }
+
+    @Test
+    public void testAllDistinct() {
+        assertArrayEquals(new int[] {3, 3}, CountDistinctElementsInWindow.countDistinct(new int[] {1, 2, 3, 4}, 3));
+    }
+
+    @Test
+    public void testWindowSizeEqualsArray() {
+        assertArrayEquals(new int[] {4}, CountDistinctElementsInWindow.countDistinct(new int[] {1, 2, 3, 4}, 4));
+    }
+
+    @Test
+    public void testInvalidInput() {
+        assertThrows(IllegalArgumentException.class, () -> CountDistinctElementsInWindow.countDistinct(new int[] {}, 2));
+    }
+}

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

@@ -0,0 +1,36 @@
+package com.thealgorithms.strings;
+// author: Vraj Prajapati @Rosander0
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class LongestCommonSubstringTest {
+
+    @Test
+    public void testNullOrEmptyInputs() {
+        assertEquals("", LongestCommonSubstring.longestCommonSubstring(null, "abc"));
+        assertEquals("", LongestCommonSubstring.longestCommonSubstring("abc", null));
+        assertEquals("", LongestCommonSubstring.longestCommonSubstring("", "abc"));
+        assertEquals("", LongestCommonSubstring.longestCommonSubstring("abc", ""));
+    }
+
+    @Test
+    public void testNormalSubstrings() {
+        assertEquals("cde", LongestCommonSubstring.longestCommonSubstring("abcdef", "zcdemf"));
+        assertEquals("abc", LongestCommonSubstring.longestCommonSubstring("abc", "abc"));
+        assertEquals("cdef", LongestCommonSubstring.longestCommonSubstring("abcdef", "cdefgh"));
+    }
+
+    @Test
+    public void testSingleCharacterAndNoMatch() {
+        assertEquals("a", LongestCommonSubstring.longestCommonSubstring("a", "a"));
+        assertEquals("", LongestCommonSubstring.longestCommonSubstring("abc", "xyz"));
+    }
+
+    @Test
+    public void testMultipleMatchesFirstLongest() {
+        // Keeps the first matched longest substring when lengths are tied
+        assertEquals("abc", LongestCommonSubstring.longestCommonSubstring("abcXdef", "abcYdef"));
+    }
+}