def reorderList(self, head: ListNode) -> None:
"""Do not return anything, modify head in-place instead.
"""
s = ListNode('')
s.next = head
# step 1: reverse the 2nd half of the linked list: 1->2->3->4->5 to 1->2->3<-4<-5
x = y = s
while y:
x = x.next
y = y.next
if y:
y = y.next
# x is the middle one, reverse from x to ende
prev, curr = None, x
while curr:
hold = curr.next
curr.next = prev
prev = curr
curr = hold
# step 2: two pointers from init and ende to switch 1(x)->2->3<-4<-5(y) to 1->5->2(x)->3<-4(y) to 1->5->2->4->3
x, y = head, prev
while not (x == y or x.next == y):
# hold of x.next and y.next
u, v = x.next, y.next
x.next = y
y.next = u
x, y = u, v
return None
def deleteNode(self, node: ListNode) -> None:
"""Do not return anything, modify node in-place instead.
Key: O(1), move the value of node.next, and link from node to node.next.next.
The key is since we don't have prev, we modify this node as prev and instead delete node.next as node.
"""
node.val = node.next.val
node.next = node.next.next
def reverseBetween(self, head: ListNode, m: int, n: int) -> ListNode:
if m == n: return head
s = ListNode('')
s.next = head
i = 0
x = y = z = s
while i < m - 1:
x = x.next
y = y.next
z = z.next
i += 1
y = y.next
z = z.next
i += 1
p = x
while i < n:
q = z.next
z.next = p
p = z
z = q
i += 1
x.next = z
y.next = z.next
z.next = p
return s.next
def rotateRight(self, head: ListNode, k: int) -> ListNode:
"""Two pointers.
"""
s = ListNode(0)
s.next = head
if not s.next:
return s.next
x, y, i = s, s, 0
while i < k:
if y.next:
y = y.next
else:
y = s.next
k %= i
if k == 0:
k += i * 2
else:
k += i
i += 1
while y.next:
x = x.next
y = y.next
if not x == s:
y.next = s.next
s.next = x.next
x.next = None
return s.next
def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
"""Q0002
"""
# parse list to str
x1 = self._parse(l1)
x2 = self._parse(l2)
# add over strs
n1 = len(x1)
n2 = len(x2)
# zip truncation
if n1 < n2:
x1 = "0" * (n2 - n1) + x1
else:
x2 = "0" * (n1 - n2) + x2
# add over strs
s = ListNode('')
k, node = 0, s
for i1, i2 in zip(x1[::-1], x2[::-1]):
v = k + int(i1) + int(i2)
k = v // 10
v %= 10
node.prev = ListNode(v)
node.prev.next = node
node = node.prev
if k:
node.prev = ListNode(k)
node.prev.next = node
node = node.prev
# sentinel
if node == s:
node = None
else:
s.prev.next = None
return node
def removeZeroSumSublists(self, head: ListNode) -> ListNode:
# linked list -> array
node, nums = head, []
while node:
nums.append(node.val)
node = node.next
# consecutive sum 0 <=> prefix sum seen
prefix, seen, remove = list(accumulate(nums)), {}, set()
for i, x in enumerate(prefix):
if x == 0:
for k in range(i + 1):
remove.add(k)
elif x in seen and seen[x] not in remove:
for k in range(seen[x] + 1, i + 1):
remove.add(k)
else:
seen[x] = i
ans = [nums[i] for i in range(len(nums)) if i not in remove]
# array -> linked list
s = prev = ListNode('')
for x in ans:
curr = ListNode(x)
prev.next = curr
prev = curr
return s.next
def deleteNode(self, node: ListNode) -> None:
"""Do not return anything, modify node in-place instead.
Key: O(N), move the value one by one while traverse from node to tail.
"""
while node.next.next:
node.val = node.next.val
node = node.next
node.val = node.next.val
node.next = None
def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
s = l = ListNode(0)
while l1 and l2:
if l1.val <= l2.val:
l.next = ListNode(l1.val)
l1 = l1.next
else:
l.next = ListNode(l2.val)
l2 = l2.next
l = l.next
l.next = l1 or l2
return s.next
def deleteDuplicates(self, head: ListNode) -> ListNode:
s = ListNode('')
s.next = head
x = y = s
while y and y.next:
while y.next and y.val == y.next.val:
y = y.next
y = y.next
if (not y) or (not y.next) or (not y.val == y.next.val):
x.next = y
x = x.next
return s.next
def deleteDuplicates(self, head: ListNode) -> ListNode:
s = ListNode(0)
s.next = head
x = y = head
while y and y.next:
y = y.next
if x.val < y.val:
x.next = y
x = x.next
if x and x.next:
x.next = None
return s.next
def removeNthFromEnd(self, head: ListNode, n: int) -> ListNode:
s = ListNode(0)
s.next = head
x = s
z = s
for i in range(n):
x = x.next
while x.next:
x = x.next
z = z.next
z.next = z.next.next
return s.next
def removeElements(self, head: ListNode, val: int) -> ListNode:
s = ListNode('')
s.next = head
prev, node = s, head
while node:
if node.val == val:
# skip node, hold prev
prev.next = node.next
else:
# move prev to forward
prev = node
node = node.next
return s.next
def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
n = s = ListNode(0)
k = 0
while l1 or l2 or k > 0:
if l1:
k += l1.val
l1 = l1.next
if l2:
k += l2.val
l2 = l2.next
n.next = ListNode(k % 10)
k //= 10
n = n.next
return s.next
def partition(self, head: ListNode, x: int) -> ListNode:
p = l = ListNode('')
q = g = ListNode('')
z = head
while z:
if z.val < x:
l.next = z
l = l.next
else:
g.next = z
g = g.next
z = z.next
l.next = q.next
g.next = None
return p.next
def reverseList(self, head: ListNode) -> ListNode:
"""Iterative Solver 1.
"""
s = ListNode(0)
s.next = head
if s.next and s.next.next:
x = s.next
y = s.next.next
while y:
h = s.next
z = y.next
s.next = y
y.next = h
x.next = z
y = x.next
return s.next
def sortList(self, head: ListNode) -> ListNode:
"""merge sort on linked list.
iterate over linked list, say, s -> head -> ...-> ..
k: number of every k nodes are sorted, k = 1, 2, .. n, e.g., s -> l -> .. -> x -> .. -> y -> .. -> r -> ..
--------- ---------
k nodes k nodes
merge k nodes from x and y, so every 2k nodes are sorted, l and r are the left and right node for next x, y pair
"""
s = ListNode('')
s.next = head
# linked list length
# it is ok to get n from k = 1 case, but this way is more cleaner
x, n = s, 0
while x.next:
x = x.next
n += 1
# merge sort
k = 1
while k < n:
l = s
while l.next:
x = l.next
# move k step forward, break link so x -> ... -> None, with lengh k
y = x
for _ in range(k - 1):
if y is not None:
y = y.next
if y is not None:
h = y
y = y.next
h.next = None
# move k step forward, break link so y -> ... -> None, with lengh k
r = y
for _ in range(k - 1):
if r is not None:
r = r.next
if r is not None:
h = r
r = r.next
h.next = None
# merge and connect, so that l -> mergeSorted(x, y) -> r
if y is not None and not x == y:
l = self.merge(l, x, y, r)
else:
break
k *= 2
return s.next
def oddEvenList(self, head: ListNode) -> ListNode:
"""TC: O(N), SC: O(1).
"""
# sentinel head for odd and even
so, se = ListNode(''), ListNode('')
xo, xe = so, se
while head:
xo.next = head
xo = xo.next
if head.next:
head = head.next
xe.next = head
xe = xe.next
head = head.next
xo.next = se.next
xe.next = None
return so.next
def plusOne(self, head: ListNode) -> ListNode:
# sentinel node
s = ListNode('')
s.next = head
# create prev along the way
node = s
while node.next:
node.next.prev = node
node = node.next
# plus one on the node from tail
while True:
if node.val < 9:
node.val += 1
break
else:
node.val = 0
node = node.prev
if node.val == '':
x = ListNode(1)
x.next = node.next
node.next.prev = x
node.next = x
x.prev = node
break
return s.next
def reverseList(self, head: ListNode) -> ListNode:
"""Recursive Solver.
"""
if head and head.next:
x = self.reverseList(head.next)
head.next.next = head
head.next = None
return x
else:
return head
def deleteNodes(self, head: ListNode, m: int, n: int) -> ListNode:
s = ListNode('')
s.next = head
prev, node = s, head
while node:
count = 0
while node and count < m:
prev, node = node, node.next
count += 1
hold = prev
if count == m:
count = 0
while node and count < n:
prev, node = node, node.next
count += 1
prev = hold
# passthrough
prev.next = node
return s.next
def insertionSortList(self, head: ListNode) -> ListNode:
# INTEGER_MIN
s = ListNode(-2147483648)
s.next = head
# insertion sort
x = head
while x and x.next:
y = x.next
if y.val < x.val:
z = s
while not z == x:
if y.val < z.next.val:
x.next = y.next
y.next = z.next
z.next = y
break
z = z.next
else:
x = x.next
return s.next
def isPalindrome(self, head: ListNode) -> bool:
"""Two pass, 1st reach to tail and create reverse link along the way, 2nd two pointers head and tail.
An alternative approach is 1st pass two pointers slow (1 step each iteration) and fast (2 step each iteration),
and when fast reach to the tail, slow reach to middle, then reverse along the way slow from middle to the tail,
and then 2nd pass two pointers head and tail and comparison along the way.
However, this alternative approach need more caution on corner cases, odd/even middle, empty linked list and etc.
"""
# 1st pass
s = ListNode('')
s.next = head
prev, node = s, head
while node:
node.prev = prev
prev = node
node = node.next
t = ListNode('')
prev.next = t
t.prev = prev
# 2nd pass
x, y = s.next, t.prev
# equality test use x is y (kind like x === y in javascript) instead of x == y, strict for reference equality,
# because in the config.listnode, class ListNode defined __eq__() as value equal for general purpose equality.
while not (x is y or x.prev is y):
if not x.val == y.val:
return False
else:
x = x.next
y = y.prev
return True
def mergeKLists(self, lists: List[ListNode]) -> ListNode:
k = len(lists)
h = [l for l in lists if l]
heapq.heapify(h)
s = ListNode(0)
x = s
while h:
v = heapq.heappop(h)
x.next = v
x = x.next
if v.next:
heapq.heappush(h, v.next)
return s.next
def reverseKGroup(self, head: ListNode, k: int) -> ListNode:
"""Keep Two Pairs, prev and curr, prev group tail and curr group head.
"""
s = prevK = ListNode(0)
s.next = holdK = currK = head
i = 1
while currK:
currK = currK.next
if i % k == 0:
prev, curr = currK, holdK
for _ in range(k):
curr.next, curr, prev = prev, curr.next, curr
prevK.next, prevK, holdK = prev, holdK, currK
i += 1
return s.next
def removeZeroSumSublists(self, head: ListNode) -> ListNode:
# O(N), one pass, in-place
# s: sentinel node
s = node = ListNode(0)
s.next = head
# consecutive sum 0 <=> prefix sum seen
prefix, seen = 0, OrderedDict()
while node:
prefix += node.val
prev = seen.get(prefix, node)
while prefix in seen:
seen.popitem()
seen[prefix] = prev
prev.next = node.next
node = node.next
return s.next
def swapPairs(self, head: ListNode) -> ListNode:
s = ListNode(0)
s.next = head
x = s
while x.next and x.next.next:
z = ListNode(x.next.val)
x.next = x.next.next
x = x.next
z.next = x.next
x.next = z
x = x.next
return s.next