new_contents: '''
  Write a program to find the node at which the intersection of two singly linked lists begins.
  
  
  For example, the following two linked lists:
  
  A:          a1 → a2
                     ↘
                       c1 → c2 → c3
                     ↗            
  B:     b1 → b2 → b3
  
  begin to intersect at node c1.
  
  Notes:
  
  If the two linked lists have no intersection at all, return null.
  The linked lists must retain their original structure after the function returns.
  You may assume there are no cycles anywhere in the entire linked structure.
  Your code should preferably run in O(n) time and use only O(1) memory.
  '''
  
  # Definition for singly-linked list.
  # class ListNode:
  #     def __init__(self, x):
  #         self.val = x
  #         self.next = None
  
  class Solution:
      # @param two ListNodes
      # @return the intersected ListNode
      def getIntersectionNode(self, headA, headB):
          if not headA or not headB: return None
          flaga = False
          pa = headA
          pb = headB 
          while True:
              if pa is pb:
                  return pa
              pa = pa.next
              pb = pb.next
              if not pa:
                  if not flaga:
                      flaga = True
                      pa = headB
                  else:
                      return None
              if not pb:
                  pb = headA
  
  
subject: Create Intersection_of_Two_Linked_Lists.py