1394. Find Lucky Integer in an Array

https://leetcode.com/problems/find-lucky-integer-in-an-array/

Given an array of integers arr, a lucky integer is an integer which has a frequency in the array equal to its value.

Return a lucky integer in the array. If there are multiple lucky integers return the largest of them. If there is no lucky integer return -1.

Example 1:

Input: arr = [2,2,3,4]
Output: 2
Explanation: The only lucky number in the array is 2 because frequency[2] == 2.

Example 2:

Input: arr = [1,2,2,3,3,3]
Output: 3
Explanation: 1, 2 and 3 are all lucky numbers, return the largest of them.

Example 3:

Input: arr = [2,2,2,3,3]
Output: -1
Explanation: There are no lucky numbers in the array.

Example 4:

Input: arr = [5]
Output: -1

Example 5:

Input: arr = [7,7,7,7,7,7,7]
Output: 7

Constraints:

• 1 <= arr.length <= 500

• 1 <= arr[i] <= 500

找数组中出现次数和值相等中最大的数，没有次数和值相等的元素时返回-1。依题意统计频率后找max。

class Solution:
    def findLucky(self, arr: List[int]) -> int:
        cnt = collections.Counter(arr)
        return max([k for k, v in cnt.items() if k == v] + [-1])