array nesting

A zero-indexed array A consisting of N different integers is given. The array contains all integers in the range [0, N - 1].

Sets S[K] for 0 <= K < N are defined as follows:

S[K] = { A[K], A[A[K]], A[A[A[K]]], ... }.

Sets S[K] are finite for each K and should NOT contain duplicates.

Write a function that given an array A consisting of N integers, return the size of the largest set S[K] for this array.

Input: A = [5,4,0,3,1,6,2] Output: 4 Explanation: A[0] = 5, A[1] = 4, A[2] = 0, A[3] = 3, A[4] = 1, A[5] = 6, A[6] = 2.

One of the longest S[K]: S[0] = {A[0], A[5], A[6], A[2]} = {5, 6, 2, 0}

Note: N is an integer within the range [1, 20,000]. The elements of A are all distinct. Each element of array A is an integer within the range [0, N-1].

解答：

class Solution(object):
    def arrayNesting(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        ans, step, n = 0, 0, len(nums)
        seen = [False] * n
        for i in range(n):
            while not seen[i]:
                seen[i] = True
                i, step = nums[i], step + 1
            ans = max(ans, step)
            step = 0
        return ans