Fix: Resolve CI failures for compression algorithms

Author: Microindole

src/main/java/com/thealgorithms/compression/RunLengthEncoding.java

@@ -75,7 +75,7 @@ public static String decompress(String compressedText) {
         for (char ch : compressedText.toCharArray()) {
             if (Character.isDigit(ch)) {
                 // Build the number for runs of 10 or more (e.g., "12A")
-                count = count * 10 + (ch - '0');
+                count = count * 10 + ch - '0';
             } else {
                 // Append the character 'count' times
                 decompressed.append(String.valueOf(ch).repeat(Math.max(0, count)));

src/main/java/com/thealgorithms/compression/ShannonFano.java

@@ -107,13 +107,10 @@ public static Map<Character, String> generateCodes(String text) {
      * @param prefix  The current prefix code being built for the symbols in this partition.
      */
     private static void buildCodeTree(List<Symbol> symbols, int start, int end, String prefix) {
-        if (start > end) {
-            return;
-        }
-
+        // The initial check in generateCodes ensures start <= end is always true here.
+        // The base case is when a partition has only one symbol.
         if (start == end) {
-            // Base case: single symbol gets the current prefix as its code.
-            symbols.get(start).code = prefix.isEmpty() ? "0" : prefix;
+            symbols.get(start).code = prefix;
             return;
         }
 

src/test/java/com/thealgorithms/compression/RunLengthEncodingTest.java

@@ -6,6 +6,15 @@
 
 class RunLengthEncodingTest {
 
+    @Test
+    void testNullInputs() {
+        // Test that a null input to compress returns an empty string
+        assertEquals("", RunLengthEncoding.compress(null));
+
+        // Test that a null input to decompress returns an empty string
+        assertEquals("", RunLengthEncoding.decompress(null));
+    }
+
     @Test
     void testCompressionSimple() {
         // Test a typical string with multiple runs

src/test/java/com/thealgorithms/compression/ShannonFanoTest.java

@@ -4,11 +4,16 @@
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.util.Map;
-
 import org.junit.jupiter.api.Test;
 
 class ShannonFanoTest {
 
+    @Test
+    void testNullInput() {
+        // Test with a null string, should return an empty map
+        assertTrue(ShannonFano.generateCodes(null).isEmpty());
+    }
+
     @Test
     void testSimpleString() {
         // A simple string to test basic code generation
@@ -21,7 +26,6 @@ void testSimpleString() {
         assertEquals("11", codes.get('C'));
     }
 
-
     @Test
     void testExampleFromStringIssue() {
         // Example from the original issue proposal: A:15, B:7, C:6, D:6, E:5