code: def findDuplicate(self, nums: List[int]) -> int:
      seen = set()
  
      for num in nums:
          if num in seen:
              return num
          else:
              seen.add(num)
  
      return -1
  
label_desc: Use image node_id 1 for calling the Solution obj's underlying member method code with example usage: obj.findDuplicate(nums) and returns: unknown, num