剑指 Offer 25. 合并两个排序的链表

# 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
        """
        # 有序-双指针
        res = dum = ListNode(None)
        while l1 and l2:
            if l1.val <= l2.val:
                res.next = l1
                l1 = l1.next              
            else:
                res.next = l2
                l2 = l2.next 
            res = res.next
        res.next = l1 if l1 else l2
        return dum.next