leetcode-21题:merge two sorted lists

def mergeTwoLists(self, l1, l2):
    """
    :type l1: ListNode
    :type l2: ListNode
    :rtype: ListNode
    """
    head = ListNode(None)
    p = head

    while l1!=None and l2!=None:
        p.next = l1
        if l1.val < l2.val:
            p = p.next
            l1 = l1.next
        else:
            p.next = l2
            p = p.next
            l2 = l2.next

    if l1 != None:
        p.next = l1
    if l2 != None:
        p.next = l2

    return head.next