test: adding subject and main

subjects/java/checkpoints/longest-common-prefix/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        LongestCommonPrefix lcp = new LongestCommonPrefix();
+
+        // Test case 1
+        String[] strs1 = {"flower", "flow", "flight"};
+        System.out.println("Longest common prefix: " + lcp.findLongestCommonPrefix(strs1)); // Expected output: "fl"
+
+        // Test case 2
+        String[] strs2 = {"dog", "racecar", "car"};
+        System.out.println("Longest common prefix: " + lcp.findLongestCommonPrefix(strs2)); // Expected output: ""
+
+        // Test case 3
+        String[] strs3 = {"interspecies", "interstellar", "interstate"};
+        System.out.println("Longest common prefix: " + lcp.findLongestCommonPrefix(strs3)); // Expected output: "inters"
+    }
+}

subjects/java/checkpoints/longest-common-prefix/README.md

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+## Longest Common Prefix
+
+### Instructions
+
+Write a function to find the longest common prefix string amongst an array of strings. If there is no common prefix, return an empty string `""`.
+
+### Expected Class
+
+```java
+public class LongestCommonPrefix {
+    public String findLongestCommonPrefix(String[] strs) {
+        // Implementation to find the longest common prefix
+    }
+}
+```
+
+### Usage
+
+Here is a possible `ExerciseRunner.java` to test your class:
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        LongestCommonPrefix lcp = new LongestCommonPrefix();
+
+        // Test case 1
+        String[] strs1 = {"flower", "flow", "flight"};
+        System.out.println("Longest common prefix: " + lcp.findLongestCommonPrefix(strs1)); // Expected output: "fl"
+
+        // Test case 2
+        String[] strs2 = {"dog", "racecar", "car"};
+        System.out.println("Longest common prefix: " + lcp.findLongestCommonPrefix(strs2)); // Expected output: ""
+
+        // Test case 3
+        String[] strs3 = {"interspecies", "interstellar", "interstate"};
+        System.out.println("Longest common prefix: " + lcp.findLongestCommonPrefix(strs3)); // Expected output: "inters"
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Longest common prefix: fl
+Longest common prefix:
+Longest common prefix: inters
+$
+```