Swap Nodes in Pairs

Problem

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

Solution

/**
 * 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 prev = dummy;
    let first = head;
    let second = head?.next;

    while (second) {
        // swap first and second
        prev.next = second;
        first.next = second.next;
        second.next = first;

        // update variables
        prev = first;
        first = first.next;
        second = first?.next;
    }
    return dummy.next;
};

Iterate through the linked list head while keeping track of three variables, the pair of nodes first and second that will be swapped, and the node prev that is directly before first. Using these, we continuously swap every two adjacent nodes until reaching the end of the list.

Problem​

Solution​

Problem

Solution