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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

# Definition for singly-linked list.
class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution:
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if l1 is None and l2 is None:
            return l1
        if l1 is None:
            return l2
        if l2 is None:
            return l1
        start = ListNode(0)
        pos = start
        while l1 and l2:
            if l1.val<l2.val:
                pos.next = l1
                l1 = l1.next
                pos = pos.next
            else:
                pos.next = l2
                l2 = l2.next
                pos = pos.next
        if l1:
            pos.next = l1
            l1 = l1.next
            pos = pos.next
        if l2:
            pos.next = l2
            l2 = l2.next
            pos = pos.next
        return start.next