Merge Two Sorted Lists

算是merge k sorted list的简单版，两个链表当前节点值进行比较，较小的新建节点然后往前移就行了，下面是代码

class Solution(object):
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        dummy = ListNode('di')
        cur = dummy
        while l1 and l2:
            if l1.val < l2.val:
                cur.next = ListNode(l1.val)
                l1 = l1.next
            else:
                cur.next = ListNode(l2.val)
                l2 = l2.next
            cur = cur.next

        cur.next = l1 if l1 else l2

        return dummy.next