Question

24. Swap Nodes in Pairs

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Description

Given a linked list, swap every two adjacent nodes and return its head. You must solve the problem without modifying the values in the list's nodes (i.e., only nodes themselves may be changed.)

 

Example 1:

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

Example 2:

Input: head = []
Output: []

Example 3:

Input: head = [1]
Output: [1]

 

Constraints:

• The number of nodes in the list is in the range [0, 100].
• 0 <= Node.val <= 100

Solutions

Solution 1: Recursion

We can implement swapping two nodes in the linked list through recursion.

The termination condition of recursion is that there are no nodes in the linked list, or there is only one node in the linked list. At this time, swapping cannot be performed, so we directly return this node.

Otherwise, we recursively swap the linked list $head.next.next$, and let the swapped head node be $t$. Then we let $p$ be the next node of $head$, and let $p$ point to $head$, and $head$ point to $t$, finally return $p$.

The time complexity is $O(n)$, and the space complexity is $O(n)$. 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 swapPairs(self, head: Optional[ListNode]) -> Optional[ListNode]:
    if head is None or head.next is None:
      return head
    t = self.swapPairs(head.next.next)
    p = head.next
    p.next = head
    head.next = t
    return p

/**
 * 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 swapPairs(ListNode head) {
    if (head == null || head.next == null) {
      return head;
    }
    ListNode t = swapPairs(head.next.next);
    ListNode p = head.next;
    p.next = head;
    head.next = t;
    return p;
  }
}

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *   int val;
 *   ListNode *next;
 *   ListNode() : val(0), next(nullptr) {}
 *   ListNode(int x) : val(x), next(nullptr) {}
 *   ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
  ListNode* swapPairs(ListNode* head) {
    if (!head || !head->next) {
      return head;
    }
    ListNode* t = swapPairs(head->next->next);
    ListNode* p = head->next;
    p->next = head;
    head->next = t;
    return p;
  }
};

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *   Val int
 *   Next *ListNode
 * }
 */
func swapPairs(head *ListNode) *ListNode {
  if head == nil || head.Next == nil {
    return head
  }
  t := swapPairs(head.Next.Next)
  p := head.Next
  p.Next = head
  head.Next = t
  return p
}

/**
 * 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 swapPairs(head: ListNode | null): ListNode | null {
  if (!head || !head.next) {
    return head;
  }
  const t = swapPairs(head.next.next);
  const p = head.next;
  p.next = head;
  head.next = t;
  return p;
}

// 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 swap_pairs(head: Option<Box<ListNode>>) -> Option<Box<ListNode>> {
    let mut dummy = Some(Box::new(ListNode { val: 0, next: head }));
    let mut cur = dummy.as_mut().unwrap();
    while cur.next.is_some() && cur.next.as_ref().unwrap().next.is_some() {
      cur.next = {
        let mut b = cur.next.as_mut().unwrap().next.take();
        cur.next.as_mut().unwrap().next = b.as_mut().unwrap().next.take();
        let a = cur.next.take();
        b.as_mut().unwrap().next = a;
        b
      };
      cur = cur.next.as_mut().unwrap().next.as_mut().unwrap();
    }
    dummy.unwrap().next
  }
}

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *   this.val = (val===undefined ? 0 : val)
 *   this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var swapPairs = function (head) {
  if (!head || !head.next) {
    return head;
  }
  const t = swapPairs(head.next.next);
  const p = head.next;
  p.next = head;
  head.next = t;
  return p;
};

# Definition for singly-linked list.
# class ListNode
#   attr_accessor :val, :next
#   def initialize(val = 0, _next = nil)
#     @val = val
#     @next = _next
#   end
# end
# @param {ListNode} head
# @return {ListNode}
def swap_pairs(head)
  dummy = ListNode.new(0, head)
  pre = dummy
  cur = head
  while !cur.nil? && !cur.next.nil?
    t = cur.next
    cur.next = t.next
    t.next = cur
    pre.next = t
    pre = cur
    cur = cur.next
  end
  dummy.next
end

Solution 2: Iteration

We set a dummy head node $dummy$, initially pointing to $head$, and then set two pointers $pre$ and $cur$, initially $pre$ points to $dummy$, and $cur$ points to $head$.

Next, we traverse the linked list. Each time we need to swap the two nodes after $pre$, so we first judge whether $cur$ and $cur.next$ are empty. If they are not empty, we perform the swap, otherwise we terminate the loop.

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 swapPairs(self, head: Optional[ListNode]) -> Optional[ListNode]:
    dummy = ListNode(next=head)
    pre, cur = dummy, head
    while cur and cur.next:
      t = cur.next
      cur.next = t.next
      t.next = cur
      pre.next = t
      pre, cur = cur, cur.next
    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 swapPairs(ListNode head) {
    ListNode dummy = new ListNode(0, head);
    ListNode pre = dummy;
    ListNode cur = head;
    while (cur != null && cur.next != null) {
      ListNode t = cur.next;
      cur.next = t.next;
      t.next = cur;
      pre.next = t;
      pre = cur;
      cur = cur.next;
    }
    return dummy.next;
  }
}

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *   int val;
 *   ListNode *next;
 *   ListNode() : val(0), next(nullptr) {}
 *   ListNode(int x) : val(x), next(nullptr) {}
 *   ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
  ListNode* swapPairs(ListNode* head) {
    ListNode* dummy = new ListNode(0, head);
    ListNode* pre = dummy;
    ListNode* cur = head;
    while (cur && cur->next) {
      ListNode* t = cur->next;
      cur->next = t->next;
      t->next = cur;
      pre->next = t;
      pre = cur;
      cur = cur->next;
    }
    return dummy->next;
  }
};

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *   Val int
 *   Next *ListNode
 * }
 */
func swapPairs(head *ListNode) *ListNode {
  dummy := &ListNode{Next: head}
  pre, cur := dummy, head
  for cur != nil && cur.Next != nil {
    t := cur.Next
    cur.Next = t.Next
    t.Next = cur
    pre.Next = t
    pre, cur = cur, cur.Next
  }
  return dummy.Next
}

/**
 * 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 swapPairs(head: ListNode | null): ListNode | null {
  const dummy = new ListNode(0, head);
  let [pre, cur] = [dummy, head];
  while (cur && cur.next) {
    const t = cur.next;
    cur.next = t.next;
    t.next = cur;
    pre.next = t;
    [pre, cur] = [cur, cur.next];
  }
  return dummy.next;
}

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *   this.val = (val===undefined ? 0 : val)
 *   this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var swapPairs = function (head) {
  const dummy = new ListNode(0, head);
  let [pre, cur] = [dummy, head];
  while (cur && cur.next) {
    const t = cur.next;
    cur.next = t.next;
    t.next = cur;
    pre.next = t;
    [pre, cur] = [cur, cur.next];
  }
  return dummy.next;
};

# 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
   * @return ListNode
   */

  function swapPairs($head) {
    $dummy = new ListNode(0);
    $dummy->next = $head;
    $prev = $dummy;

    while ($head !== null && $head->next !== null) {
      $first = $head;
      $second = $head->next;

      $first->next = $second->next;
      $second->next = $first;
      $prev->next = $second;

      $prev = $first;
      $head = $first->next;
    }

    return $dummy->next;
  }
}