text: '''
  class ListNode:
      def __init__(self, x):
          self.val = x
          self.next = None
  '''
  class Solution:
      #given two list nodes
      #return one list node
      def mergeTwoLists(self, l1, l2):
          dummy = ListNode(0)
          p = dummy
          while l1 and l2:
              if l1.val < l2.val:
                  p.next = l1
                  l1 = l1.next
              else:
                  p.next = l2
                  l2 = l2.next
              p = p.next
          if l1:
              p.next = l1
          elif l2:
              p.next = l2
          return dummy.next
  