puts "What year do you want to start?"
start_year = gets.chomp 
puts "What year do you want to end ?"
end_year = gets.chomp

if start_year.to_i > end_year.to_i  
    puts "Starting year should smaller than ending year!"
else
    puts "leap year between " + start_year + " and " + end_year +" :"
end

while start_year.to_i < end_year.to_i
    while ((start_year.to_i%4 == 0 and start_year.to_i%100 != 0) or (start_year.to_i%100==0 and start_year.to_i%400 == 0))
        puts start_year
        start_year = start_year.to_i + 1 # 请问这里应该怎样理解呢？
    end
    start_year = start_year.to_i + 1 # 请问这里应该怎样理解呢？
end