Add LongestCommonPrefix (#5849)

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

@@ -0,0 +1,73 @@
+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class LongestCommonPrefixTest {
+
+    private final LongestCommonPrefix longestCommonPrefix = new LongestCommonPrefix();
+
+    @Test
+    public void testCommonPrefix() {
+        String[] input = {"flower", "flow", "flight"};
+        String expected = "fl";
+        assertEquals(expected, longestCommonPrefix.longestCommonPrefix(input));
+    }
+
+    @Test
+    public void testNoCommonPrefix() {
+        String[] input = {"dog", "racecar", "car"};
+        String expected = "";
+        assertEquals(expected, longestCommonPrefix.longestCommonPrefix(input));
+    }
+
+    @Test
+    public void testEmptyArray() {
+        String[] input = {};
+        String expected = "";
+        assertEquals(expected, longestCommonPrefix.longestCommonPrefix(input));
+    }
+
+    @Test
+    public void testNullArray() {
+        String[] input = null;
+        String expected = "";
+        assertEquals(expected, longestCommonPrefix.longestCommonPrefix(input));
+    }
+
+    @Test
+    public void testSingleString() {
+        String[] input = {"single"};
+        String expected = "single";
+        assertEquals(expected, longestCommonPrefix.longestCommonPrefix(input));
+    }
+
+    @Test
+    public void testCommonPrefixWithDifferentLengths() {
+        String[] input = {"ab", "a"};
+        String expected = "a";
+        assertEquals(expected, longestCommonPrefix.longestCommonPrefix(input));
+    }
+
+    @Test
+    public void testAllSameStrings() {
+        String[] input = {"test", "test", "test"};
+        String expected = "test";
+        assertEquals(expected, longestCommonPrefix.longestCommonPrefix(input));
+    }
+
+    @Test
+    public void testPrefixAtEnd() {
+        String[] input = {"abcde", "abcfgh", "abcmnop"};
+        String expected = "abc";
+        assertEquals(expected, longestCommonPrefix.longestCommonPrefix(input));
+    }
+
+    @Test
+    public void testMixedCase() {
+        String[] input = {"Flower", "flow", "flight"};
+        String expected = "";
+        assertEquals(expected, longestCommonPrefix.longestCommonPrefix(input));
+    }
+}