def ans(A): w = set() # walked res = 0 # result for a in A: c = 0 # count while a not in w: w.add(a) c += 1 a = A[a] res = max(res, c) print(res) # 0 1 2 3 4 5 6 7 8 9 ans([5,4,0,3,1,2]) ans([5,4,0,3,1,2,9,6,7,8])