LeetCode： Find Lucky Integer in an Array

Question

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.

Solution #1

def find_lucky(arr)
  dict = Hash.new(0)
  arr.each do |element|
    dict[element] += 1
  end
  dict.delete_if { |k, v| k != v }
  return -1 if dict.empty?
  dict.values.max
end