Loop Solution: Swap Nodes in Pairs

Loop solution: notice the loop condition is node != null && node.next != null, and set the preNode.next to new swapped node too!

/**
 * 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) {
        if(head == null || head.next == null){
            return head;
        }
        ListNode newHead = new ListNode(Integer.MIN_VALUE);
        newHead.next = head;
        ListNode preNode = newHead;
        ListNode node = preNode.next;
        while(node != null && node.next != null){
            ListNode temp = node.next;
            preNode.next = temp;
            node.next = temp.next;
            temp.next = node;
            preNode = node;
            node = node.next;
        }
        return newHead.next;
    }
}