https://oj.leetcode.com/problems/swap-nodes-in-pairs/

Swap Nodes in Pairs.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode p = head;
        ListNode q;
        ListNode r;
        ListNode ret;
        if ((p == null) || (p.next == null)){
            return head;
        }else{
            ret = head.next;
            q = head.next.next;
            head.next = q;
            ret.next = head;
            p = head;
            q = head.next;
            while((q != null) && (q.next != null)){
                r = q.next.next;
                p.next = q.next;
                q.next = r;
                p.next.next = q;
                
                p = p.next.next;
                q = q.next;
            }
        }
        return ret;
    }
}