python第8周作业——leetcode 21 合并两个有序链表

题目：

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：
用归并排序把有序链表合并。可用递归方法与非递归方法。

方法1：递归

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

class Solution:
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if l1==None and l2==None:
            return None
        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

方法2：非递归方法：

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

class Solution:
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        head = ListNode(0)
        first = head
        while l1!=None and l2!=None:
            if l1.val <= l2.val:
                head.next = l1
                l1 = l1.next
            else:
                head.next = l2
                l2 = l2.next
            head = head.next
        if l1 != None:
            head.next = l1
        elif l2 != None:
            head.next = l2
        return first.next