把题目讲清楚，
让演示更从容。

合并两个有序链表

给定两个升序链表，将它们合并为一个新的升序链表。

def merge(left, right):
    dummy = ListNode(0)
    current = dummy
    while left and right:
        if left.val <= right.val:
            current.next = left
            left = left.next
        else:
            current.next = right
            right = right.next
        current = current.next
    current.next = left or right
    return dummy.next