Array Nesting 问题及解法

问题描述：

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.

示例:

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

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

问题分析：

在数组中找环，已经遍历过的值可弃用。不再参与遍历，最后找到环长度最长的即为答案。

过程详见代码：

class Solution {public:    int arrayNesting(vector<int>& nums) {        vector<bool> used(nums.size(),false);int res = 0;for (int i = 0; i < nums.size(); i++){if (used[i]) continue;used[i] = true;int start = i;int end = nums[start];int count = 1;while (end != start){used[end] = true;end = nums[end];count++;}res = max(res,count);}return res;    }};