Swap Nodes in Pairs

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

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

public class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode safe = new ListNode(-1);
        ListNode pre = safe;
        safe.next = head;
        ListNode cur = head;
        if(head==null || head.next==null) return head;
        ListNode post = head.next;
        while(post!=null){
            ListNode temp = post.next;
            post.next = cur;
            cur.next = temp;
            pre.next = post;
            pre = cur;
            cur = temp;
            post = temp==null?null:temp.next;
        }
        return safe.next;
    }
}