[LeetCode]题解（python）：021-Merge Two Sorted Lists

题目来源：

　　https://leetcode.com/problems/merge-two-sorted-lists/

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题意分析：

　　题目给出两个排好序的链表，将这两个链表整合成一个新的有序的链表。

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题目思路：

　　这道题目很简单，首先构造一个新的链表，比较两个链表的指针指向的节点的值大小，将值较少的节点放到新链表中，并将指针位置往后移动一位，直到某个链表为空之后，把剩下的链表全部放到新的链表中就可以了。时间复杂度是（O（m+n））

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代码（python）：

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

 class Solution(object):
     def mergeTwoLists(self, l1, l2):
         """
         :type l1: ListNode
         :type l2: ListNode
         :rtype: ListNode
         """
         ans = ListNode(0)
         tmp = ans
         if l1 == None and l2 == None:
             return None
         while l1 !=None or l2 != None:
             if l1 == None:
                 while l2 != None:
                     tmp.val = l2.val
                     l2 = l2.next
                     if l2 == None:
                         break
                     tmp.next = ListNode(0)
                     tmp = tmp.next
                 break
             if l2 == None:
                 while l1 != None:
                     tmp.val = l1.val
                     l1 = l1.next
                     if l1 == None:
                         break
                     tmp.next = ListNode(0)
                     tmp = tmp.next
                 break
             if l1.val <= l2.val:
                 tmp.val = l1.val
                 l1 = l1.next
             else:
                 tmp.val = l2.val
                 l2 = l2.next
             tmp.next = ListNode(0)
             tmp = tmp.next
         return ans

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