24. 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.

两个一组，保存好前驱，交换完毕继续推进。开头也要换，所以增加一个冗余节点作为辅助。

public class Solution {
   public static ListNode swapPairs(ListNode head)
	{
		if(head==null)
			return head;
		
		ListNode dummy=new ListNode(6);
		dummy.next=head;
		ListNode p1=head;
		ListNode p2=head.next;
		ListNode prev=dummy;
		
		while(p2!=null)
		{
			p1.next=p2.next;
			p2.next=p1;
			prev.next=p2;
			prev=p1;
			p1=p1.next;
			if(p1!=null)
				p2=p1.next;
			else {
				break;
			}
		}
		
		return dummy.next;
	}
}

还有递归方法：

https://discuss.leetcode.com/topic/4351/my-accepted-java-code-used-recursion

public class Solution {
    public ListNode swapPairs(ListNode head) {
        if ((head == null)||(head.next == null))
            return head;
        ListNode n = head.next;
        head.next = swapPairs(head.next.next);
        n.next = head;
        return n;
    }
}