21. Merge Two Sorted Lists

太傻了 。。总之上代码：

# 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:
            return l2
        elif l2 == None:
            return l1
        dump = ListNode(0)
        head = dump
        while l1  and  l2:
            if l1.val < l2.val:
                head.next = ListNode(l1.val)
                head = head.next
                l1 = l1.next
            else:
                head.next = ListNode(l2.val)
                head = head.next
                l2 = l2.next
        if l1 :
            head.next = l1
        elif l2:
            head.next = l2
        else:
            head.next = None
        return dump.next