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.

第一遍：

public class Solution {
    public ListNode swapPairs(ListNode head) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        if(head == null || head.next == null) return head;
        ListNode header = new ListNode(-1), first = head, second = head.next, cur = header;
        header. next = head;
        while(second != null){
            first.next = second.next;
            second.next = first;
            cur.next = second;
            cur = first;
            first = cur.next;
            if(first != null)
                second = first.next;
            else
                second = null;
        }
        return header.next;
    }
}

第三遍：

public class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode header = new ListNode(-1);
        header.next = head;
        ListNode cur = header;
        while(cur.next != null && cur.next.next != null){
            ListNode first = cur.next.next, second = cur.next;
            second.next = first.next;
            cur.next = first;
            first.next = second;
            cur = second;
        }
        return header.next;
    }
}