[leetcode笔记] 21. 合并两个有序链表

题目描述

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

示例：

输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4

解题思路

1. 用递归的方法解决
2. 用循环的方法解决

代码

用递归的方法：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

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

用循环的方法：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        l4 = l3 = ListNode(None)
        while l1 and l2:
            if l1.val<=l2.val:
                l3.next = l1
                l1 = l1.next
            else:
                l3.next = l2
                l2 = l2.next
            l3 = l3.next
        if l1:
            l3.next = l1
        elif l2:
            l3.next = l2
        return l4.next