I am trying to sort a list of given elements by decreasing frequency. If two elements have the same frequency, they should appear in increasing order. For example, given the input: [6, 1000, 3, 3, 1000, 6, 6, 6]
the output should be: [6, 6, 6, 6, 3, 3, 1000, 1000]
. The elements also must be sorted in place rather than returning a new list.
So far, I have created a HashMap of the key and values, where the key is the element and the value is the frequency. But I'm not quite sure what to do next:
public static void method(List<Integer> items)
{
int size = items.size();
int count = 0;
HashMap<Integer, Integer> map = new HashMap<>();
for (int i = 0; i < size; ++i)
{
int item = items.get(i);
if (map.containsKey(item))
{
map.put(item, map.get(item) + 1);
}
else
{
map.put(item, 1);
}
}
List list = new LinkedList(map.entrySet());
Collections.sort(list, new Comparator()
{
public int compare(Object o1, Object o2)
{
return ((Comparable) ((Map.Entry) (o1)).getValue())
.compareTo(((Map.Entry) (o2)).getValue());
}
});
HashMap sortedMap = new LinkedHashMap();
for (Iterator it = list.iterator(); it.hasNext();)
{
Map.Entry entry = (Map.Entry) it.next();
sortedMap.put(entry.getKey(), entry.getValue());
}
}
You can sort inline as follows :
public static void sortInline(List<Integer> list) {
Map<Integer, Long> map = list.stream()
.collect(Collectors.groupingBy(Function.identity(),
Collectors.counting())); // frequency map
Comparator<Integer> frequencyComparison = Comparator
.<Integer>comparingLong(map::get).reversed(); // sort the entries by value in reverse order
list.sort(frequencyComparison.thenComparing(Comparator.naturalOrder())); // then by key for the collisions
}