565.数组嵌套

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

首先想到的是直接暴力遍历,以数组中每一个元素为起点，看最远能走多远：

from typing import List

class Solution:
    # 暴力
    def arrayNesting(self, nums: List[int]) -> int:
        result = 0
        for x in nums:
            next_n = x
            s = set()
            while next_n not in s:
                s.add(next_n)
                next_n = nums[next_n]

            if len(s) > result:
                result = len(s)

        return result

一顿操作，直接超时。

思考一下，直接遍历会有很多重复的操作。
如果某个元素已经出现过了，那么就不用再看它了。

from typing import List

class Solution:
    # 暴力
    def arrayNesting(self, nums: List[int]) -> int:
        has_appeared = set()
        result = 0
        for x in nums:
            if x in has_appeared:
                continue
            next_n = x
            s = set()

            while next_n not in s:
                has_appeared.add(next_n)
                s.add(next_n)
                next_n = nums[next_n]
            if len(s) > result:
                result = len(s)

        return result