def reverse_list(head: ListNode) -> ListNode:
if not head or not head.next:
return head
last = reverse_list(head.next)
head.next.next = head
head.next = None
return last
def ls():
lns = {x: ListNode(x) for x in range(1, 5)}
lns[1].next = lns[2]
lns[2].next = lns[3]
lns[3].next = lns[4]
return lns[1]
def ans():
lns = {x: ListNode(x) for x in range(1, 6)}
lns[1].next = lns[4]
lns[4].next = lns[3]
lns[3].next = lns[2]
lns[2].next = lns[5]
return lns[1]
def swap_pairs(head: ListNode) -> ListNode:
if not head or not head.next:
return head
dummy = ListNode()
pre, cur, nxt = None, head, head.next
dummy.next = nxt
while cur and nxt:
if pre:
pre.next = nxt
cur.next, nxt.next = nxt.next, cur
if cur.next:
pre, cur, nxt = cur, cur.next, cur.next.next
else:
break
return dummy.next
def reverse_n(head: ListNode, n: int) -> ListNode:
if n == 1:
successor[0] = head.next
return head
last = reverse_n(head.next, n - 1)
head.next.next = head
head.next = successor[0]
return last
def ls():
lns = {x: ListNode(x) for x in range(1, 7)}
lns[1].next = lns[2]
lns[2].next = lns[3]
lns[4].next = lns[5]
lns[5].next = lns[6]
return lns[1], lns[4]
def add_two_numbers(l1: ListNode, l2: ListNode) -> ListNode:
dummy = ListNode()
curr = dummy
p1, p2 = l1, l2
carrier = 0
while p1 or p2:
x = p1.val if p1 else 0
y = p2.val if p2 else 0
carrier, val = divmod(x + y + carrier, 10)
curr.next = ListNode(val)
curr = curr.next
if p1:
p1 = p1.next
if p2:
p2 = p2.next
if carrier > 0:
curr.next = ListNode(carrier)
return dummy.next
def reverse_kgroup(head: ListNode, k: int) -> ListNode:
count = 0
cur = head
while cur:
cur = cur.next
count += 1
if count < k:
return head
pre, new_head = reverse(head, k)
head.next = reverse_kgroup(new_head, k)
return pre
def reverse_between(head: ListNode, m: int, n: int) -> ListNode:
if m == 1:
return reverse_n_wrapper(head, n)
head.next = reverse_between(head.next, m - 1, n - 1)
return head
def ans():
lns = {x: ListNode(x) for x in [5, 7, 9]}
lns[5].next = lns[7]
lns[7].next = lns[9]
return lns[5]