leetcode -- Swap Nodes in Pairs

双指针互换，要考虑一些边界条件，比如链表为空，链表长度为1，链表长度为2.
加一个safeGuard可以避开链表长度为2的检测。

/**
 * 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) {
        // Start typing your Java solution below
        // DO NOT write main() function
        
        if(head == null)
            return null;
            
        ListNode safeG = new ListNode(-1);
        safeG.next = head;
        ListNode before = safeG;
        ListNode cur = safeG.next;
        ListNode after = cur.next;
        
        while(after != null){
            ListNode tmp = after.next;
            after.next = cur;
            cur.next = tmp;
            before.next = after;
            before = cur;
            if(before.next == null)
                break;
                
            cur = before.next;
            after = cur.next;
        }
        
        head = safeG.next;
        return head;
        
    }
}