Merge two sorted lists

Merge two sorted lists into one sorted list. The first half of the row is list A, the second half is list B.

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def merge(a, b):
    dummy = tail = ListNode()
    while a and b:
        if a.val <= b.val: tail.next, a = a, a.next
        else: tail.next, b = b, b.next
        tail = tail.next
    tail.next = a or b
    return dummy.next