leetcode21. 合并两个有序链表

思路：
用一个新链表来表示合并后的有序链表，
每次比较两个链表，将较小的那个结点存储至新链表中

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def mergeTwoLists(self, list1, list2):
        """
        :type list1: Optional[ListNode]
        :type list2: Optional[ListNode]
        :rtype: Optional[ListNode]
        """
        if list1 is None:
            if list2 is None:
                return None
            else:
                return list2
        else:
            if list2 is None:
                return list1
        preList =ListNode(0)
        p = preList
        while list1 and list2:
            if list1.val<=list2.val:
                p.next=list1
                list1=list1.next
            else:
                p.next=list2
                list2=list2.next
            p =p.next
        if list1 is None:
            p.next=list2
        else:
            p.next=list1
        return preList.next