21. Merge Two Sorted Lists-Python

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

思路

递归最容易实现，根据l1和l2头节点数据域的大小，决定合并后链表的头结点，这样问题的规模减1，递归直到满足其中的3个返回条件的中的一个：

if l1==None and l2==None:

return None

if l1==None:

return l2

if l2==None:

return l1

代码实现

# 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
"""
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