quick_sort.java

public void quickSort(int[] arr) {
  if (arr == null || arr.length <= 1) return;
  
  quickSort(arr, 0, arr.length);
}

public void quickSort(int[] arr, int s, int e) {
  if (arr.length == 0 || s >= e) return;
  
  int sepIndex = partition(arr, s, e);
  quickSort(arr, s, sepIndex);
  quickSort(arr, sepIndex + 1, e);
}

public int partition(int[] arr, int s, int e) {
  if (s == e) {
    return s;
  }
  
  int pivot = arr[e-1];
  int i = s;
  int j = s;
  while (i < e-1) {
    int val = arr[i];
    if (val < pivot) {
      swap(arr, i, j);
      j++;
    }
    i++;
  }
  swap(arr, e-1, j);
  return j;
}

public void swap(int[] arr, int i, int j){
      int tmp = arr[j];
      arr[j] = arr[i];
      arr[i] = tmp;
}