Approach: Dummy + relink pairs

def swapPairs(self, head):
    dummy = ListNode(0, head)
    prev = dummy
    while prev.next and prev.next.next:
        a = prev.next
        b = a.next
        a.next = b.next
        b.next = a
        prev.next = b
        prev = a
    return dummy.next

Complexity: O(n) time, O(1) space.

Full Solution — all languages

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next
class Solution:
    def swapPairs(self, head):
        dummy = ListNode(0, head)
        prev = dummy
        while prev.next and prev.next.next:
            a = prev.next
            b = a.next
            a.next = b.next
            b.next = a
            prev.next = b
            prev = a
        return dummy.next

import java.util.*;
class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
    }
}
class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode prev = dummy;
        while (prev.next != null && prev.next.next != null) {
            ListNode a = prev.next, b = a.next;
            a.next = b.next;
            b.next = a;
            prev.next = b;
            prev = a;
        }
        return dummy.next;
    }
}

#include <bits/stdc++.h> using namespace std;
struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x): val(x), next(nullptr) {
    }
};
class Solution {
    public: ListNode* swapPairs(ListNode* head) {
        ListNode dummy(0);
        dummy.next = head;
        ListNode* prev = &dummy;
        while (prev->next && prev->next->next) {
            ListNode* a = prev->next;
            ListNode* b = a->next;
            a->next = b->next;
            b->next = a;
            prev->next = b;
            prev = a;
        }
        return dummy.next;
    }
};

/** Node shape: { val, next }. Build/return plain objects or use this ctor. */ 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 prev = dummy;
    while (prev.next && prev.next.next) {
        const a = prev.next, b = a.next;
        a.next = b.next;
        b.next = a;
        prev.next = b;
        prev = a;
    }
    return dummy.next;
};

Swap Nodes in Pairs

Example 1

Example 2

Constraints