Added MapReduce Algorithm in Misc Folder. (#4828)

* Added MapReduce Algorithm in Misc Folder.

* Did formatting correctly

* Removed main function and added MapReduceTest

* format the code

src/main/java/com/thealgorithms/misc/MapReduce.java

@@ -0,0 +1,39 @@
+package com.thealgorithms.misc;
+
+import java.util.Arrays;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
+/*
+* MapReduce is a programming model for processing and generating large data sets with a parallel,
+distributed algorithm on a cluster.
+* It has two main steps: the Map step, where the data is divided into smaller chunks and processed in parallel,
+and the Reduce step, where the results from the Map step are combined to produce the final output.
+* Wikipedia link : https://en.wikipedia.org/wiki/MapReduce
+*/
+
+public class MapReduce {
+    /*
+     *Counting all the words frequency within a sentence.
+     */
+    public static String mapreduce(String sentence) {
+        List<String> wordList = Arrays.stream(sentence.split(" ")).toList();
+
+        // Map step
+        Map<String, Long> wordCounts = wordList.stream().collect(Collectors.groupingBy(Function.identity(), LinkedHashMap::new, Collectors.counting()));
+
+        // Reduce step
+        StringBuilder result = new StringBuilder();
+        wordCounts.forEach((word, count) -> result.append(word).append(": ").append(count).append(","));
+
+        // Removing the last ',' if it exists
+        if (!result.isEmpty()) {
+            result.setLength(result.length() - 1);
+        }
+
+        return result.toString();
+    }
+}