原题链接：https://leetcode.com/problems/merge-two-sorted-lists/
解题思路：把两个node的元素从头开始比较，哪一个小，便在结果中加入该node，然后往后移一位。如果哪一个node先遍历完了，就把剩下的node剩下的元素全部扔进result里。
代码：

class Solution(object):
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        point1 = l1
        point2 = l2
        result = ListNode(0)
        head = result
        while point1 or point2:
            if not point1:
                result.next = point2
                break
            if not point2:
                result.next = point1
                break
            if point1.val <= point2.val:
                result.next = point1
                result = result.next
                point1 = point1.next
            else:
                result.next = point2
                result = result.next
                point2 = point2.next
        return head.next