solution.rb

class Solution
    # @param a : array of integers
    # @return an integer
    def solve(a)
        found = -1
        
        sorted = a.select { |n| n >= 0 }.sort
        s = sorted.size
        sorted.each_with_index do |n, i|
            if n >= 0 && n == s - 1 - i && sorted[i+1] != n
                found = 1
                break
            end
        end
        
        return found
    end
end