# 24. Swap Nodes in Pairs

## 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.

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.

• /**
* 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 {
ListNode dummy = new ListNode(0, head);
ListNode pre = dummy;
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;
}
}

• /**
* 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* dummy = new ListNode(0, head);
ListNode* pre = dummy;
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.
# 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]:
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


• /**
* type ListNode struct {
*     Val int
*     Next *ListNode
* }
*/
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
}

• /**
* 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;
}


• /**
* function ListNode(val, next) {
*     this.val = (val===undefined ? 0 : val)
*     this.next = (next===undefined ? null : next)
* }
*/
/**
* @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
#     attr_accessor :val, :next
#     def initialize(val = 0, _next = nil)
#         @val = val
#         @next = _next
#     end
# end
# @return {ListNode}
pre = dummy
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

• // 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.
# 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;
}
}