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.

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

class Solution(object):
    def mergeTwoLists(self, a, b):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if a and b:
            if a.val > b.val:
                a, b = b, a
            a.next = self.mergeTwoLists(a.next, b)
        return a or b

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