func getIntersectionNode(headA, headB *ListNode) *ListNode {
    if headA==nil || headB==nil{
        return nil
    }
    pa,pb := headA,headB
    for pa!=pb {
        if pa!=nil{
            pa = pa.Next
        }else {
            pa = headB
        }

        if pb!=nil{
            pb = pb.Next
        }else {
            pb = headA
        }
    }
    return pa
}