021. Merge Two Sorted Lists.py

# 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
        """
        dummyHead = ListNode(None)
        tail = dummyHead

        while l1 or l2:
            if l1 and (not l2 or l1.val<l2.val):
                tail.next = l1
                l1 = l1.next
            else:
                tail.next = l2
                l2 = l2.next

            tail = tail.next

        return dummyHead.next