25. Reverse Nodes in k-Group

Approach 1: Recursive

• Time: $O(n)$
• Space: $O(n)$

C++JavaPython

class Solution {
 public:
  ListNode* reverseKGroup(ListNode* head, int k) {
    if (head == nullptr)
      return nullptr;

    ListNode* tail = head;

    for (int i = 0; i < k; ++i) {
      // There are less than k nodes in the list, do nothing.
      if (tail == nullptr)
        return head;
      tail = tail->next;
    }

    ListNode* newHead = reverse(head, tail);
    head->next = reverseKGroup(tail, k);
    return newHead;
  }

 private:
  // Reverses [head, tail).
  ListNode* reverse(ListNode* head, ListNode* tail) {
    ListNode* prev = nullptr;
    ListNode* curr = head;
    while (curr != tail) {
      ListNode* next = curr->next;
      curr->next = prev;
      prev = curr;
      curr = next;
    }
    return prev;
  }
};

class Solution {
  public ListNode reverseKGroup(ListNode head, int k) {
    if (head == null)
      return null;

    ListNode tail = head;

    for (int i = 0; i < k; ++i) {
      // There are less than k nodes in the list, do nothing.
      if (tail == null)
        return head;
      tail = tail.next;
    }

    ListNode newHead = reverse(head, tail);
    head.next = reverseKGroup(tail, k);
    return newHead;
  }

  // Reverses [head, tail).
  private ListNode reverse(ListNode head, ListNode tail) {
    ListNode prev = null;
    ListNode curr = head;
    while (curr != tail) {
      ListNode next = curr.next;
      curr.next = prev;
      prev = curr;
      curr = next;
    }
    return prev;
  }
}

class Solution:
  def reverseKGroup(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:
    if not head:
      return None

    tail = head

    for _ in range(k):
      # There are less than k nodes in the list, do nothing.
      if not tail:
        return head
      tail = tail.next

    newHead = self._reverse(head, tail)
    head.next = self.reverseKGroup(tail, k)
    return newHead

  def _reverse(self, head: Optional[ListNode], tail: Optional[ListNode]) -> Optional[ListNode]:
    """Reverses [head, tail)."""
    prev = None
    curr = head
    while curr != tail:
      next = curr.next
      curr.next = prev
      prev = curr
      curr = next
    return prev

Approach 2: Iterative

• Time: $O(n)$
• Space: $O(1)$

C++JavaPython

class Solution {
 public:
  ListNode* reverseKGroup(ListNode* head, int k) {
    if (!head || k == 1)
      return head;

    const int length = getLength(head);
    ListNode dummy(0, head);
    ListNode* prev = &dummy;
    ListNode* curr = head;

    for (int i = 0; i < length / k; ++i) {
      for (int j = 0; j < k - 1; ++j) {
        ListNode* next = curr->next;
        curr->next = next->next;
        next->next = prev->next;
        prev->next = next;
      }
      prev = curr;
      curr = curr->next;
    }

    return dummy.next;
  }

 private:
  int getLength(ListNode* head) {
    int length = 0;
    for (ListNode* curr = head; curr; curr = curr->next)
      ++length;
    return length;
  }
};

class Solution {
  public ListNode reverseKGroup(ListNode head, int k) {
    if (head == null || k == 1)
      return head;

    final int length = getLength(head);
    ListNode dummy = new ListNode(0, head);
    ListNode prev = dummy;
    ListNode curr = head;

    for (int i = 0; i < length / k; ++i) {
      for (int j = 0; j < k - 1; ++j) {
        ListNode next = curr.next;
        curr.next = next.next;
        next.next = prev.next;
        prev.next = next;
      }
      prev = curr;
      curr = curr.next;
    }

    return dummy.next;
  }

  private int getLength(ListNode head) {
    int length = 0;
    for (ListNode curr = head; curr != null; curr = curr.next)
      ++length;
    return length;
  }
}

class Solution:
  def reverseKGroup(self, head: ListNode, k: int) -> ListNode:
    if not head or k == 1:
      return head

    def getLength(head: ListNode) -> int:
      length = 0
      while head:
        length += 1
        head = head.next
      return length

    length = getLength(head)
    dummy = ListNode(0, head)
    prev = dummy
    curr = head

    for _ in range(length // k):
      for _ in range(k - 1):
        next = curr.next
        curr.next = next.next
        next.next = prev.next
        prev.next = next
      prev = curr
      curr = curr.next

    return dummy.next