Array Nesting

Question

A zero-indexed array A of length N contains all integers from 0 to N-1. Find and return the longest length of set S, where S[i] = {A[i], A[A[i]], A[A[A[i]]], … } subjected to the rule below.

Suppose the first element in S starts with the selection of element A[i] of index = i, the next element in S should be A[A[i]], and then A[A[A[i]]]… By that analogy, we stop adding right before a duplicate element occurs in S.

Example 1:

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}

LeetCodeleetcode.com/problems/array-nesting

Solution

/*
@param: int[]
@return: int
Algorithm: 一开始用set存一个cycle，后来发现只需要记录起点index就OK，避免重复遍历，把遍历过的修改为-1；
*/
public int arrayNesting(int[] nums) {
    if (nums.length == 0) return 0;
    
    int n = nums.length;
    int res = 0;
    
    for (int i = 0; i < n; i++) {
        if (nums[i] == -1) continue;
        int j = nums[i];
        int count = 1;
        while (j != i) {
            int temp = nums[j];
            nums[j] = -1;
            j = temp;
            count++;
        }
        res = Math.max(res, count);            
    }
    return res;
}