Approach: Two-pointer gap of k

def swapNodes(self, head, k):
    first = head
    for _ in range(k - 1):
        first = first.next
    second = head
    cur = first
    while cur.next:
        cur = cur.next
        second = second.next
    first.val, second.val = second.val, first.val
    return head

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 swapNodes(self, head, k):
        first = head
        for _ in range(k - 1):
            first = first.next
        second = head
        cur = first
        while cur.next:
            cur = cur.next
            second = second.next
        first.val, second.val = second.val, first.val
        return head

import java.util.*;
class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
    }
}
class Solution {
    public ListNode swapNodes(ListNode head, int k) {
        ListNode first = head;
        for (int i = 0; i < k - 1; i++) first = first.next;
        ListNode second = head, cur = first;
        while (cur.next != null) {
            cur = cur.next;
            second = second.next;
        }
        int t = first.val;
        first.val = second.val;
        second.val = t;
        return head;
    }
}

#include <bits/stdc++.h> using namespace std;
struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x): val(x), next(nullptr) {
    }
};
class Solution {
    public: ListNode* swapNodes(ListNode* head, int k) {
        ListNode* first = head;
        for (int i = 0; i < k - 1; i++) first = first->next;
        ListNode* second = head;
        ListNode* cur = first;
        while (cur->next) {
            cur = cur->next;
            second = second->next;
        }
        int t = first->val;
        first->val = second->val;
        second->val = t;
        return head;
    }
};

/** 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
 * @param {number} k
 * @return {ListNode}
 */ var swapNodes = function(head, k) {
    let first = head;
    for (let i = 0; i < k - 1; i++) first = first.next;
    let second = head, cur = first;
    while (cur.next) {
        cur = cur.next;
        second = second.next;
    }
    const t = first.val;
    first.val = second.val;
    second.val = t;
    return head;
};

Swapping Nodes in a Linked List

Example 1

Example 2

Constraints