Solution for Reversing a Map in Java

challenges/reverse/solutions/ReverseAMap.java

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+//  https://github.com/WomenWhoCodeNYC/Algorithms/blob/master/challenges/reverse/reverse.md
+
+/*
+
+Level 6
+
+Create a function called reverse() that takes in a set of key-value pairs
+(object in Javascript, hash in Ruby, dictionary in Python, etc.) as the input and returns an object
+with the same keys and values inverted. If multiple keys have the same value, it should assign of array of keys.
+
+Example: inputObject: {'a':1, 'b':2}
+
+reverse(inputObject) should return {1:'b', 2:'c'}
+
+inputObject2 = { "a": "1", "b": "2", "c": "1", "d": "3" }
+
+reverse(inputObject2) should return { "1": ["a", "c"], "2": ["b"] "3": ["d"] }
+
+
+*/
+
+import java.util.*;
+import static java.util.Map.Entry;
+
+public class ReverseAMap {
+
+    // imperative style of coding
+    public static Map reverseMapImperatively(Map<String, String> inputMap){
+        Map<String, List<String>> outputMap = new HashMap<>();
+        String newKey, newValue;
+        List<String> list;
+        for(Entry<String, String> entry : inputMap.entrySet()) {
+            newKey = entry.getValue();
+            newValue = entry.getKey();
+            if(outputMap.containsKey(newKey))
+                list = outputMap.get(newKey);
+            else {
+                list = new ArrayList<String>();
+                outputMap.put(newKey, list);
+            }
+            list.add(newValue);
+        }
+        return outputMap;
+    }
+
+    // declarative style of coding
+    public static Map reverseMapDeclaratively(Map<String, String> inputMap) {
+        Map<String, List<String>> outputMap = new HashMap<>();
+        inputMap.forEach((key,value) -> outputMap.merge(value, List.of(key), ReverseAMap::mergeListsOfValues));
+        return outputMap;
+    }
+
+    private static List<String> mergeListsOfValues(List<String> key1, List<String> key2) {
+        List<String> valuesList = new ArrayList<>(key1);
+        valuesList.addAll(key2);
+        return valuesList;
+        /*
+            // if need the merged list to be in a sorted order
+            Set<String> set = new TreeSet<>(key1);
+            set.addAll(key2);
+            return new ArrayList<>(set);
+        */
+    }
+
+    public static void main(String[] args) {
+        Map<String,String> map = Map.of("a","1", "b","2", "c", "1", "d", "3", "as", "3");   // Java 9
+        System.out.println(reverseMapImperatively(map));
+        System.out.println(reverseMapDeclaratively(map));
+    }
+}