LeetCode 24. Swap Nodes in Pairs (javascript)

Given a linked list, swap every two adjacent nodes and return its head.

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) {
    if (head === null) return null;
    if (head.next === null) return head;
    let dummy = head;
    while (head !== null && head.next !== null) {
        swap(head);
        if (head.next !== null)
            head = head.next.next;
    }
    return dummy;
};

function swap(head) {
    [head.val, head.next.val] = [head.next.val, head.val];
    return head;
}