Swap Nodes in Pairs; Data Structure; Linked List; Pointer;

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head == null || head.next == null) return head;
        ListNode nex = head.next.next;
        ListNode newHead = head.next;
        head.next.next = head;
        head.next = nex;
        ListNode prev = head;
        ListNode cur = nex;
        while(cur != null && cur.next != null){
            nex = cur.next.next;
            prev.next = cur.next;
            cur.next.next = cur;
            cur.next = nex;
            prev = cur;
            cur = nex;
        }
        return newHead;
    }
}