请问这道题返回值为什么是none？？？

Question

class Solution:         def intersect(self, nums1, nums2):                 if len(nums1) < len(nums2):                         self.so(nums1,nums2)                 else:                         self.so(nums2,nums1)                  def so(self,nums1,nums2):                 list1 = []                 for i in range(len(nums1)):                         if nums1[i] in nums2:                                 list1.append(nums1[i])                                 nums2.remove(nums1[i])                 return list1     def main():         xx = Solution()         b = xx.intersect([1,2,2,1],[2,2])         print(type(b))         print(b) if __name__ == '__main__':     main()