LC 21合并两个有序链表Merge Two Sorted Lists　简单

将两条升序链表合并为一条升序链表并返回头节点。

思路 哑结点 + 双指针每次摘较小头节点尾插，余链直接接上。时间 O(m+n)。

class Solution:
    def mergeTwoLists(self, l1, l2):
        dummy = cur = ListNode()
        while l1 and l2:
            if l1.val <= l2.val:
                cur.next = l1
                l1 = l1.next
            else:
                cur.next = l2
                l2 = l2.next
            cur = cur.next
        cur.next = l1 if l1 else l2
        return dummy.next