0565. Array Nesting

https://leetcode.com/problems/array-nesting

Description

You are given an integer array nums of length n where nums is a permutation of the numbers in the range [0, n - 1].

You should build a set s[k] = {nums[k], nums[nums[k]], nums[nums[nums[k]]], ... } subjected to the following rule:

• The first element in s[k] starts with the selection of the element nums[k] of index = k.

• The next element in s[k] should be nums[nums[k]], and then nums[nums[nums[k]]], and so on.

• We stop adding right before a duplicate element occurs in s[k].

Return the longest length of a set s[k].

Example 1:

**Input:** nums = [5,4,0,3,1,6,2]
**Output:** 4
**Explanation:** 
nums[0] = 5, nums[1] = 4, nums[2] = 0, nums[3] = 3, nums[4] = 1, nums[5] = 6, nums[6] = 2.
One of the longest sets s[k]:
s[0] = {nums[0], nums[5], nums[6], nums[2]} = {5, 6, 2, 0}

Example 2:

**Input:** nums = [0,1,2]
**Output:** 1

Constraints:

• 1 <= nums.length <= 105

• 0 <= nums[i] < nums.length

• All the values of nums are unique.

ac

class Solution {
    public int arrayNesting(int[] nums) {
        Set<Integer> set = new HashSet<>();
        int max = 0;
        for (int i = 0; i < nums.length; i++) {
            int cnt = 0;
            int j = i;
            while (set.add(nums[j])) {
                cnt++;
                j = nums[j];
            }
            max = Math.max(max, cnt);
        }

        return max;
    }
}

/*
1) start from each i, add visited elements to set; 2) after visiting, update max length;
*/