find K maximum value from an unsorted array(implement min heap)

Maintain a min-heap with size = k, to collect the result.

 1 //Find K minimum values from an unsorted array

 2 //Implement Min-Heap

 3 

 4 public int[] findKMax(int[] arr, int k){

 5     if(arr == null || arr.length == 0) return null;

 6     int[] result = new int[k];

 7     for(int i = 0; i < k; i ++)

 8         result[i] = arr[i];

 9     buildMinHeap(result);

10     for(int i = k; i < arr.length; i ++){

11         if(arr[i] > result[0]){

12             result[0] = arr[i];

13             minHeapify(result, 0);

14         }

15     }

16     return result;

17 }

18 

19 public void buildMinHeap(int[] arr){

20     for(int i = arr.length / 2 - 1; i > -1; i --){//bottom-up build min heap

21         minHeapify(arr, i);

22     }

23 }

24 

25 public void minHeapify(int[] arr, int curIndex){

26     int left = curIndex * 2 + 1;

27     int right = curIndex * 2 + 2;

28     int smallest = curIndex;

29     if(left < arr.length && arr[left] < arr[smallest])

30         smallest = left;

31     if(right < arr.length && arr[right] < arr[smallest])

32         smallest = right;

33     if(smallest != curIndex){

34         swap(arr, smallest, curIndex);

35         minHeapify(arr,smallest);

36     }

37 }

38 

39 public void swap(int[] arr, int aa, int bb){

40     int tmp = arr[aa];

41     arr[aa] = arr[bb];

42     arr[bb] = tmp;

43 }