Add LongestCommonPrefix.java

DIRECTORY.md

@@ -686,6 +686,7 @@
             * [Isomorphic](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/strings/Isomorphic.java)
             * [KMP](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/strings/KMP.java)
             * [LetterCombinationsOfPhoneNumber](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/strings/LetterCombinationsOfPhoneNumber.java)
+            * [LongestCommonPrefix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/strings/LongestCommonPrefix.java)
             * [LongestNonRepetitiveSubstring](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/strings/LongestNonRepetitiveSubstring.java)
             * [LongestPalindromicSubstring](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/strings/LongestPalindromicSubstring.java)
             * [Lower](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/strings/Lower.java)
@@ -1311,6 +1312,7 @@
             * [HorspoolSearchTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/HorspoolSearchTest.java)
             * [IsomorphicTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/IsomorphicTest.java)
             * [LetterCombinationsOfPhoneNumberTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/LetterCombinationsOfPhoneNumberTest.java)
+            * [LongestCommonPrefixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/LongestCommonPrefixTest.java)
             * [LongestNonRepetitiveSubstringTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/LongestNonRepetitiveSubstringTest.java)
             * [LongestPalindromicSubstringTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/LongestPalindromicSubstringTest.java)
             * [LowerTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/LowerTest.java)

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

@@ -0,0 +1,22 @@
+package com.thealgorithms.strings;
+
+import java.util.Arrays;
+
+public final class LongestCommonPrefix {
+    public String longestCommonPrefix(String[] strs) {
+        if (strs == null || strs.length == 0) {
+            return "";
+        }
+
+        Arrays.sort(strs);
+        String shortest = strs[0];
+        String longest = strs[strs.length - 1];
+
+        int index = 0;
+        while (index < shortest.length() && index < longest.length() && shortest.charAt(index) == longest.charAt(index)) {
+            index++;
+        }
+
+        return shortest.substring(0, index);
+    }
+}

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));
+    }
+}