6.9.7  Checking for a permutation: is_permu

A permutation can be written as a list, but not every list corresponds to a permutation. The is_permu is a boolean function which checks to see if a given list is a permutation.

• is_permu takes one argument:
  L, a list.
• is_permu(L) returns 1 if L is a permutation and returns 0 if L is not a permutation.

Examples.

• Input:
  is_permu([2,1,3])
  Output:

  0

• Input:
  is_permu([2,1,3,0])
  Output:

  1