Lecture Notes on 16 November 2009

def mergeSort (a, lo, hi):
  if (lo < hi):
    mid = (lo + hi) / 2
    mergeSort (a, lo, mid)
    mergeSort (a, mid + 1, hi)
    merge (a, lo, hi)

def merge (a, lo, hi):
  tmp = []
  mid = (lo + hi) / 2
  left1 = lo
  right1 = mid
  left2 = mid + 1
  right2 = hi

  while ((left1 <= right1) and (left2 <= right2)):
    if (a[left1] < a[left2]):
      tmp.append(a[left1])
      left1 = left1 + 1
    else:
      tmp.append(a[left2])
      left2 = left2 + 1

  while (left1 <= right1):
    tmp.append(a[left1])
    left1 = left1 + 1

  while (left2 <= right2):
    tmp.append(a[left2])
    left2 = left2 + 1

  aIdx = lo
  for i in range (len(tmp)):
    a[aIdx] = tmp[i]
    aIdx = aIdx + 1

def main():
  a = [9, 1, 8, 2, 7, 3, 6, 4, 5]
  print a
  mergeSort (a, 0, len(a) - 1)
  print a

main()

# Perform permutation

def permute (a, lo, hi):
  if (lo == hi):
    print a
  else:
    for i in range (lo, hi + 1):
      a[lo], a[i] = a[i], a[lo]
      permute (a, lo + 1, hi)
      a[lo], a[i] = a[i], a[lo]

def main():
  a = [1, 2, 3]
  permute (a, 0, len(a) - 1)

main()