24. Swap Nodes in Pairs

2020-01-11

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

You may not modify the values in the list’s nodes, only nodes itself may be changed.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode dummy = new ListNode(42);
        dummy.next = head;
        ListNode c = dummy;
        while (c.next != null && c.next.next != null) {
            ListNode a = c.next;
            ListNode b = c.next.next;

            // swaps nodes in pair
            c.next = b;
            a.next = b.next;
            b.next = a;
            
            c = c.next.next;
        }
        return dummy.next;
    }
}