# Python O(n) Solution without additional memory

• ``````class Solution(object):
def reverse(self, l):
if l is None or l.next is None:
return l
p1, p2, l.next = l, l.next, None
while(p2 is not None):
tmp, p2.next = p2.next, p1
p1, p2 = p2, tmp
return p1

"""
:type l1: ListNode
:type l2: ListNode
:rtype: ListNode
"""

# align two lists using two pointers
p1, p2, flag = l2, l1, 0
while (flag<2):
if p1.next is None:
p1 = l1
flag += 1
else:
p1 = p1.next
if p2.next is None:
p2 = l2
flag += 1
else:
p2 = p2.next

# initialize: deep copy the longer list
pp = l1 if p1!=l1 else l2
pp3, p3, l3 = None, None, None
while (pp is not None):
if l3 is None:
l3 = ListNode(pp.val)
pp3 = l3
else:
pp3.next = ListNode(pp.val)
pp3 = pp3.next
if pp==p1 or pp==p2:
p3 = pp3
pp = pp.next

pp1, pp2, pp3 = p1, p2, p3
while (pp1 is not None):
pp3.val = pp1.val + pp2.val
pp1, pp2, pp3 = pp1.next, pp2.next, pp3.next

pp = l3

rl3 = self.reverse(l3)
pp = rl3
carry, p = 0, rl3
while (p.next is not None):
carry, p.val = divmod(p.val+carry, 10)
p = p.next
carry, p.val = divmod(p.val+carry, 10)
p.next = ListNode(carry) if carry != 0 else None
l3 = self.reverse(rl3)
return l3
``````

• well done, but reversing the lists is not allowed.

• Hi @Yuukir , I didn't reverse the input lists.

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