Find the element with highest occurrences in an array [java]

Update:

• As Maxim pointed out, using HashMap would be a more appropriate choice than Hashtable here.
• The assumption here is that you are not concerned with concurrency. If synchronized access is needed, use ConcurrentHashMap instead.

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You can use a HashMap to count the occurrences of each unique element in your double array, and that would:

• Run in linear O(n) time, and
• Require O(n) space

Psuedo code would be something like this:

• Iterate through all of the elements of your array once: O(n)
  • For each element visited, check to see if its key already exists in the HashMap: O(1), amortized
  • If it does not (first time seeing this element), then add it to your HashMap as [key: this element, value: 1]. O(1)
  • If it does exist, then increment the value corresponding to the key by 1. O(1), amortized
• Having finished building your HashMap, iterate through the map and find the key with the highest associated value - and that's the element with the highest occurrence. O(n)

A partial code solution to give you an idea how to use HashMap:

import java.util.HashMap;
...

    HashMap hm = new HashMap();
    for (int i = 0; i < array.length; i++) {
        Double key = new Double(array[i]);
        if ( hm.containsKey(key) ) {
            value = hm.get(key);
            hm.put(key, value + 1);
        } else {
            hm.put(key, 1);
        }
    }

I'll leave as an exercise for how to iterate through the HashMap afterwards to find the key with the highest value; but if you get stuck, just add another comment and I'll get you more hints =)

Use Collections.frequency option:

 List<String> list = Arrays.asList("1", "1","1","1","1","1","5","5","12","12","12","12","12","12","12","12","12","12","8");
      int max = 0;
      int curr = 0;
      String currKey =  null;
      Set<String> unique = new HashSet<String>(list);

          for (String key : unique) {
                curr = Collections.frequency(list, key);

               if(max < curr){
                 max = curr;
                 currKey = key;
                }
            }

          System.out.println("The number "  + currKey + " happens " + max + " times");

Output:

The number 12 happens 10 times

The solution with Java 8

       int result = Arrays.stream(array)
           .boxed()
           .collect(Collectors.groupingBy(i->i,Collectors.counting()))
           .values()
           .stream()
           .max(Comparator.comparingLong(i->i))
           .orElseThrow(RuntimeException::new));