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.

对于这个题交换节点主要分为两步进行，如交换2和1，现将2删除之后，再将2插入到1与4之间，为了避免出现删除头结点的情况，所以我们在头结点前增加一个节点。成对交换就需要利用一个count来记录当前成对情况，代码如下所示：

/**
 * Created by shuang on 2017/4/1.
 */
public class SwapNodesinPairs {
    public ListNode swapPairs(ListNode head) {
        ListNode head1 = new ListNode(0);
        head1.next = head;
        ListNode head2 = head1;
        int count = 1;
        while(head2!=null&&head2.next!=null&&head2.next.next!=null){
            if(count%2 == 1){
                ListNode tmp = new ListNode(head2.next.val);
                head2.next = head2.next.next;
                tmp.next = head2.next.next;
                head2.next.next = tmp;
            }
            count++;
            head2 = head2.next;
        }
        return head1.next;
    }
}