6.9.8  Checking for a cycle: is_cycle

The is_cycle command is a boolean function which checks to see if a list represents a cycle.

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

Examples.

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

  1

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

  0