Partition algorithm

partition.py

def partition(a, lo, hi):
  pivot = a[lo]
  left = lo + 1
  right = hi 
  while left <= right:
    while a[left] < pivot:
      left += 1
    while a[right] > pivot:
      right -= 1
    if left <= right:
      a[left], a[right] = a[right], a[left]
      left += 1
      right -= 1
    a[lo], a[right] = a[right], a[lo]
    return right