【Leetcode刷题Python】21. 合并两个有序链表

1 题目

将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。

2 解析

（1）方法一：递归

（2）方法二：迭代

3 Python实现

class Solution:
    # 方法一
    '''
    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
    '''
    # 方法二：迭代法
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        head = ListNode(-1)
        prev = head
        while l1 and l2 :
            if l1.val <=l2.val:
                prev.next = l1
                l1 = l1.next
            else:
                prev.next = l2
                l2 = l2.next

            prev = prev.next
        prev.next = l1 if l1 is not None else l2
        return head.next