LeetCode每日一题——T21. 合并两个有序链表（易）：链表，递归 / 迭代

法一：递归

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        if l1 is None:
            return l2
        elif l2 is None:
            return l1
        elif l1.val < l2.val:
            l1.next = self.mergeTwoLists(l1.next, l2)
            return l1
        else:
            l2.next = self.mergeTwoLists(l1, l2.next)
            return l2

法二：迭代

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        res_head = ListNode(-2)			
        # 因为链表无头节点，所以自己创建头节点，其中 -2 无任何意义；头结点 -->next 指向第一个节点
        
        res = res_head		# 尾节点
        
        while l1 and l2:
            # 尾节点的 next 指向 “min(l1, l2)”
            if l1.val < l2.val:
                res.next = l1
                l1 = l1.next
            else:
                res.next = l2
                l2 = l2.next
            res = res.next		# 尾节点指针指向 “min(l1, l2)”，链表尾部添加节点成功
        
        res.next = l1 if l1 is not None else l2
        
        return res_head.next