21. Merge Two Sorted Lists

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.

AC代码：

class Solution(object):
    def mergeTwoLists(self, l1, l2):
        ret_list = current = ListNode(0)
        while l1 is not None and l2 is not None:
            if l1.val <= l2.val:
                current.next = l1
                l1 = l1.next
            else:
                current.next = l2
                l2 = l2.next
            current = current.next
        if l1 is not None:
            current.next = l1
        if l2 is not None:
            current.next = l2
        return ret_list.next

因为都是有序的链表，直接按照大小遍历即可。