Question

25. Reverse Nodes in k-Group

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Description

Given the head of a linked list, reverse the nodes of the list k at a time, and return _the modified list_.

k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes, in the end, should remain as it is.

You may not alter the values in the list's nodes, only nodes themselves may be changed.

 

Example 1:

Input: head = [1,2,3,4,5], k = 2
Output: [2,1,4,3,5]

Example 2:

Input: head = [1,2,3,4,5], k = 3
Output: [3,2,1,4,5]

 

Constraints:

• The number of nodes in the list is n.
• 1 <= k <= n <= 5000
• 0 <= Node.val <= 1000

 

Follow-up: Can you solve the problem in O(1) extra memory space?

Solutions

Solution 1: Iteration

The time complexity is $O(n)$, and the space complexity is $O(1)$. Here, $n$ is the length of the linked list.

# Definition for singly-linked list.
# class ListNode:
#   def __init__(self, val=0, next=None):
#     self.val = val
#     self.next = next
class Solution:
  def reverseKGroup(self, head: ListNode, k: int) -> ListNode:
    def reverseList(head):
      pre, p = None, head
      while p:
        q = p.next
        p.next = pre
        pre = p
        p = q
      return pre

    dummy = ListNode(next=head)
    pre = cur = dummy
    while cur.next:
      for _ in range(k):
        cur = cur.next
        if cur is None:
          return dummy.next
      t = cur.next
      cur.next = None
      start = pre.next
      pre.next = reverseList(start)
      start.next = t
      pre = start
      cur = pre
    return dummy.next

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *   int val;
 *   ListNode next;
 *   ListNode() {}
 *   ListNode(int val) { this.val = val; }
 *   ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
  public ListNode reverseKGroup(ListNode head, int k) {
    ListNode dummy = new ListNode(0, head);
    ListNode pre = dummy, cur = dummy;
    while (cur.next != null) {
      for (int i = 0; i < k && cur != null; ++i) {
        cur = cur.next;
      }
      if (cur == null) {
        return dummy.next;
      }
      ListNode t = cur.next;
      cur.next = null;
      ListNode start = pre.next;
      pre.next = reverseList(start);
      start.next = t;
      pre = start;
      cur = pre;
    }
    return dummy.next;
  }

  private ListNode reverseList(ListNode head) {
    ListNode pre = null, p = head;
    while (p != null) {
      ListNode q = p.next;
      p.next = pre;
      pre = p;
      p = q;
    }
    return pre;
  }
}

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *   Val int
 *   Next *ListNode
 * }
 */
func reverseKGroup(head *ListNode, k int) *ListNode {
  var dummy *ListNode = &ListNode{}
  p, cur := dummy, head
  for cur != nil {
    start := cur
    for i := 0; i < k; i++ {
      if cur == nil {
        p.Next = start
        return dummy.Next
      }
      cur = cur.Next
    }
    p.Next, p = reverse(start, cur), start
  }
  return dummy.Next
}

func reverse(start, end *ListNode) *ListNode {
  var pre *ListNode = nil
  for start != end {
    tmp := start.Next
    start.Next, pre = pre, start
    start = tmp
  }
  return pre
}

/**
 * Definition for singly-linked list.
 * class ListNode {
 *   val: number
 *   next: ListNode | null
 *   constructor(val?: number, next?: ListNode | null) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 *   }
 * }
 */

function reverseKGroup(head: ListNode | null, k: number): ListNode | null {
  let dummy = new ListNode(0, head);
  let pre = dummy;
  // pre->head-> ... ->tail-> next
  while (head != null) {
    let tail = pre;
    for (let i = 0; i < k; ++i) {
      tail = tail.next;
      if (tail == null) {
        return dummy.next;
      }
    }
    let t = tail.next;
    [head, tail] = reverse(head, tail);
    // set next
    pre.next = head;
    tail.next = t;
    // set new pre and new head
    pre = tail;
    head = t;
  }
  return dummy.next;
}

function reverse(head: ListNode, tail: ListNode) {
  let cur = head;
  let pre = tail.next;
  // head -> next -> ... -> tail -> pre
  while (pre != tail) {
    let t = cur.next;
    cur.next = pre;
    pre = cur;
    cur = t;
  }
  return [tail, head];
}

// Definition for singly-linked list.
// #[derive(PartialEq, Eq, Clone, Debug)]
// pub struct ListNode {
//   pub val: i32,
//   pub next: Option<Box<ListNode>>
// }
//
// impl ListNode {
//   #[inline]
//   fn new(val: i32) -> Self {
//   ListNode {
//     next: None,
//     val
//   }
//   }
// }
impl Solution {
  pub fn reverse_k_group(head: Option<Box<ListNode>>, k: i32) -> Option<Box<ListNode>> {
    fn reverse(head: Option<Box<ListNode>>) -> Option<Box<ListNode>> {
      let mut head = head;
      let mut pre = None;
      while let Some(mut node) = head {
        head = node.next.take();
        node.next = pre.take();
        pre = Some(node);
      }
      pre
    }

    let mut dummy = Some(Box::new(ListNode::new(0)));
    let mut pre = &mut dummy;
    let mut cur = head;
    while cur.is_some() {
      let mut q = &mut cur;
      for _ in 0..k - 1 {
        if q.is_none() {
          break;
        }
        q = &mut q.as_mut().unwrap().next;
      }
      if q.is_none() {
        pre.as_mut().unwrap().next = cur;
        return dummy.unwrap().next;
      }

      let b = q.as_mut().unwrap().next.take();
      pre.as_mut().unwrap().next = reverse(cur);
      while pre.is_some() && pre.as_mut().unwrap().next.is_some() {
        pre = &mut pre.as_mut().unwrap().next;
      }
      cur = b;
    }
    dummy.unwrap().next
  }
}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *   public int val;
 *   public ListNode next;
 *   public ListNode(int val=0, ListNode next=null) {
 *     this.val = val;
 *     this.next = next;
 *   }
 * }
 */
public class Solution {
  public ListNode ReverseKGroup(ListNode head, int k) {
    ListNode dummy = new ListNode(0, head);
    ListNode pre = dummy, cur = dummy;
    while (cur.next != null)
    {
      for (int i = 0; i < k && cur != null; ++i)
      {
        cur = cur.next;
      }
      if (cur == null)
      {
        return dummy.next;
      }
      ListNode t = cur.next;
      cur.next = null;
      ListNode start = pre.next;
      pre.next = ReverseList(start);
      start.next = t;
      pre = start;
      cur = pre;
    }
    return dummy.next;
  }

  private ListNode ReverseList(ListNode head) {
    ListNode pre = null, p = head;
    while (p != null)
    {
      ListNode q = p.next;
      p.next = pre;
      pre = p;
      p = q;
    }
    return pre;
  }
}

# Definition for singly-linked list.
# class ListNode {
#   public $val;
#   public $next;
#   public function __construct($val = 0, $next = null)
#   {
#     $this->val = $val;
#     $this->next = $next;
#   }
# }

class Solution {
  /**
   * @param ListNode $head
   * @param int $k
   * @return ListNode
   */

  function reverseKGroup($head, $k) {
    $dummy = new ListNode(0);
    $dummy->next = $head;
    $prevGroupTail = $dummy;

    while ($head !== null) {
      $count = 0;
      $groupHead = $head;
      $groupTail = $head;

      while ($count < $k && $head !== null) {
        $head = $head->next;
        $count++;
      }
      if ($count < $k) {
        $prevGroupTail->next = $groupHead;
        break;
      }

      $prev = null;
      for ($i = 0; $i < $k; $i++) {
        $next = $groupHead->next;
        $groupHead->next = $prev;
        $prev = $groupHead;
        $groupHead = $next;
      }
      $prevGroupTail->next = $prev;
      $prevGroupTail = $groupTail;
    }

    return $dummy->next;
  }
}

Solution 2: Recursion

The time complexity is $O(n)$, and the space complexity is $O(\log_k n)$. Here, $n$ is the length of the linked list.

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *   Val int
 *   Next *ListNode
 * }
 */
func reverseKGroup(head *ListNode, k int) *ListNode {
  start, end := head, head
  for i := 0; i < k; i++ {
    if end == nil {
      return head
    }
    end = end.Next
  }
  res := reverse(start, end)
  start.Next = reverseKGroup(end, k)
  return res
}

func reverse(start, end *ListNode) *ListNode {
  var pre *ListNode = nil
  for start != end {
    tmp := start.Next
    start.Next, pre = pre, start
    start = tmp
  }
  return pre
}

/**
 * Definition for singly-linked list.
 * class ListNode {
 *   val: number
 *   next: ListNode | null
 *   constructor(val?: number, next?: ListNode | null) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 *   }
 * }
 */

function reverseKGroup(head: ListNode | null, k: number): ListNode | null {
  if (k === 1) {
    return head;
  }

  const dummy = new ListNode(0, head);
  let root = dummy;
  while (root != null) {
    let pre = root;
    let cur = root;

    let count = 0;
    while (count !== k) {
      count++;
      cur = cur.next;
      if (cur == null) {
        return dummy.next;
      }
    }

    const nextRoot = pre.next;
    pre.next = cur;

    let node = nextRoot;
    let next = node.next;
    node.next = cur.next;
    while (node != cur) {
      [next.next, node, next] = [node, next, next.next];
    }
    root = nextRoot;
  }

  return dummy.next;
}