Add Run-Length Encoding (fixes #3911) (#3916)

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

@@ -0,0 +1,60 @@
+package com.thealgorithms.strings;
+/*  References : https://en.wikipedia.org/wiki/Run-length_encoding
+ * String compression algorithm deals with encoding the string, that is, shortening the size of the string
+ * @author Swarga-codes (https://github.com/Swarga-codes)
+*/
+public class StringCompression {
+  /**
+   * Returns the compressed or encoded string
+   * 
+   * @param ch character array that contains the group of characters to be encoded
+   * @return the compressed character array as string
+   */
+  public static String compress(String input) {
+    // Keeping the count as 1 since every element present will have atleast a count
+    // of 1
+    int count = 1;
+    String compressedString = "";
+    // Base condition to check whether the array is of size 1, if it is then we
+    // return the array
+    if (input.length() == 1) {
+      return "" + input.charAt(0);
+    }
+    // If the array has a length greater than 1 we move into this loop
+    for (int i = 0; i < input.length() - 1; i++) {
+      // here we check for similarity of the adjacent elements and change the count
+      // accordingly
+      if (input.charAt(i) == input.charAt(i + 1)) {
+        count = count + 1;
+      }
+      if ((i + 1) == input.length() - 1 && input.charAt(i + 1) == input.charAt(i)) {
+        compressedString = appendCount(compressedString, count, input.charAt(i));
+        break;
+      } else if (input.charAt(i) != input.charAt(i+1)) {
+        if ((i + 1) == input.length() - 1) {
+          compressedString = appendCount(compressedString, count, input.charAt(i)) + input.charAt(i+1);
+          break;
+        } else {
+          compressedString = appendCount(compressedString, count, input.charAt(i));
+          count = 1;
+        }
+      }
+    }
+    return compressedString;
+  }
+  /**
+   * @param res   the resulting string
+   * @param count current count
+   * @param ch    the character at a particular index
+   * @return the res string appended with the count
+   */
+  public static String appendCount(String res, int count, char ch) {
+    if (count > 1) {
+      res += ch + "" + count;
+      count = 1;
+    } else {
+      res += ch + "";
+    }
+    return res;
+  }
+}

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

@@ -0,0 +1,13 @@
+package com.thealgorithms.strings;
+import static org.junit.jupiter.api.Assertions.*;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+public class StringCompressionTest {
+    @ParameterizedTest
+    @CsvSource({"a,a","aabbb,a2b3","abbbc,ab3c","aabccd,a2bc2d"})
+    void stringCompressionTest(String input,String expectedOutput){
+        String output=StringCompression.compress(input);
+        assertEquals(expectedOutput, output);
+    }
+}