Decomposition as a product of disjoint cycles :
permu2cycles

permu2cycles takes as argument a permutation.
permu2cycles returns its decomposition as a product of disjoint cycles.
Input :

permu2cycles([1,3,4,5,2,0])

Output :

[[0,1,3,5],[2,4]]

In the answer the cycles of size 1 are omitted, except if n - 1 is a fixed point of the permutation (this is required to find the value of n from the cycle decomposition).
Input :

permu2cycles([0,1,2,4,3,5])

Output :

[[5],[3,4]]

Input :

permu2cycles([0,1,2,3,5,4])

Output :

[[4,5]]