1394-find-lucky-integer-in-an-array.py

"""
Problem Link: 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
"""
class Solution:
    def findLucky(self, arr: List[int]) -> int:
        count = {}
        for number in arr:
          count[number] = count.get(number, 0) + 1
        
        res = -1
        for key, value in count.items():
          if key  == value and key > res:
            res = key
        return res