srik sri srik sri - 10 days ago 6
Python Question

merging two sorted linked lists into one linked list in python

here is my code:

def merge_lists(head1, head2):
if head1 is None and head2 is None:
return None
if head1 is None:
return head2
if head2 is None:
return head1
if head1.value < head2.value:
temp = head1
else:
temp = head2
while head1 != None and head2 != None:
if head1.value < head2.value:
temp.next = head1
head1 = head1.next
else:
temp.next = head2
head2 = head2.next
if head1 is None:
temp.next = head2
else:
temp.next = head1
return temp
pass


the problem here is stucked in the infinite loop.can any one tell me what the problem is

the examples are:

assert [] == merge_lists([],[])
assert [1,2,3] == merge_lists([1,2,3], [])
assert [1,2,3] == merge_lists([], [1,2,3])
assert [1,1,2,2,3,3,4,5] == merge_lists([1,2,3], [1,2,3,4,5])

Answer

The problem with the current code is that it causes a side-effect of the temp node's next before it navigates to the next node from the current node. This is problematic when the current temp node is the current node.

That is, imagine this case:

temp = N
temp.next = N  # which means N.next = N
N = N.next     # but from above N = (N.next = N) -> N = N

There is a corrected version, with some other updates:

def merge_lists(head1, head2):
    if head1 is None:
        return head2
    if head2 is None:
        return head1

    # create dummy node to avoid additional checks in loop
    s = t = node() 
    while not (head1 is None or head2 is None):
        if head1.value < head2.value:
            # remember current low-node
            c = head1
            # follow ->next
            head1 = head1.next
        else:
            # remember current low-node
            c = head2
            # follow ->next
            head2 = head2.next

        # only mutate the node AFTER we have followed ->next
        t.next = c          
        # and make sure we also advance the temp
        t = t.next

    t.next = head1 or head2

    # return tail of dummy node
    return s.next
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