删除出现次数超过“N”次问题的每个元素

Question

I'm kind of stuck with a small bug in my code and I hope you could help me!

I have an ArrayList and I want to remove all the elements that repeat more than 'N' times. My solution works just fine in most cases, but when it comes to N=1 and ArrayList size is more than 30 or so it skips one element so it's not unique. Its index is random( i mean I tried to find a special index where the problem is hidden, but I could not).

Here comes the code

while(isSorted!=true) {
for (int i = 0; i < test.size(); i++) {
    counter = 0;
    temp = test.get(i);
    for (int j = i + 1; j < test.size(); j++) {
        if (temp == test.get(j)) {
            counter++;
            if (counter >= maxOccurrences) {
                test.remove(j);
               }
            }
          }
        if(counter<maxOccurrences){
           isSorted=true;
       }
    }
 }

If I just repeat this block of code one more time it deletes this element and everything is fine. I'm trying to understand why it would skip one not unique element. I could use HashSet for this particular case(when N=1) but it feels like cheating a little bit. I hope I was clear with my question and provided all information that is needed. Thank you in advance. Peace and love!

Answer 1

Messing with a List while you're iterating over it can be hazardous to your health. I suspect that your index-based loops are getting fooled by calls to remove , which cause shifts.

Your algorithm is something on the order of N^3. Consider sorting the array first if you're allowed to do so, or using additional storage to reduce search times by counting all the array elements in a single pass as suggested by another respondent.

Answer 2

It seems to me that using HashSet would be better in terms of asymptotics, since it would not exceed O(n) by the sum rule.

Map<Integer, Integer> newMap = new HashMap<>();
        integers.forEach(integer -> {
            if(newMap.containsKey(integer) && integer != null)
                newMap.put(integer, newMap.get(integer) + 1);
            else
                newMap.put(integer, 1);
        });
    return newMap.entrySet()
                .stream()
                .filter(x->x.getValue().equals(numberOfDuplicates))
                .map(x->x.getKey())
                .collect(Collectors.toList())

And what about your code it is also better to use equals() instead of == , because if the numbers are greater than 127 and less than -127 , your code will always work incorrectly.

The problem is that with remove , you need to reduce j by 1, because if the elements go in a row, the next element will move to the past j , and you won't delete it, because you're already on j+1 element

if (counter >= maxOccurrences) {
     test.remove(j);
     j--;
}