Quick Sort

Given an array of integers, sort the elements in the array in ascending order. The quick sort algorithm should be used to solve this problem.

Examples

• {1} is sorted to {1}
• {1, 2, 3} is sorted to {1, 2, 3}
• {3, 2, 1} is sorted to {1, 2, 3}
• {4, 2, -3, 6, 1} is sorted to {-3, 1, 2, 4, 6}

Corner Cases

• What if the given array is null? In this case, we do not need to do anything.
• What if the given array is of length zero? In this case, we do not need to do anything.

Time Complexity: O(NlogN), worst case O(N^2)

Space Complexity: O(N)

class Solution(object):
  def quickSort(self, array):
    """
    input: int[] array
    return: int[]
    """
    # write your solution here
    if array is None or len(array) <= 1:
      return array
    self.helper(array, 0, len(array) - 1)
    return array
    
  def helper(self, array, left, right):
    if left >= right:
      return
    num = self.partition(array, left, right)
    self.helper(array, left, num - 1)
    self.helper(array, num + 1, right)

  def partition(self, array, left, right):
    import random
    rand = random.randint(left, right)
    pivot = array[rand]
    array[rand], array[right] = array[right], array[rand]
    right_bound = right - 1
    while left <= right_bound:
      if array[left] <= pivot:
        left += 1
      elif array[right_bound] >= pivot:
        right_bound -= 1
      else:
        array[left], array[right_bound] = array[right_bound], array[left]
        left += 1
        right_bound -= 1
    array[right], array[left] = array[left], array[right]
    return left