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.

题目要求从头算起，两个结点为一组交换位置。当时的思路是先计算组数，然后在一个for循环中组内交换。再一想不用记录组数，直接在循环内p=p.next.next，直接跳过两个就可以了。而且实现交换的条件也简单，需要 p!=null && p.next !=null。这样可以解决如果结点个数是奇数的情况。

代码：

package leetcode;

import java.util.List;

public class SwapNodes {
	public static void main(String[] args) {
		SwapNodes s = new SwapNodes();
		ListNode n1 = new ListNode(1);
		ListNode n2 = new ListNode(2);
		ListNode n3 = new ListNode(3);
		ListNode n4 = new ListNode(4);
		n1.next=n2;
		n2.next=n3;
		n3.next=n4;
		
		 s.swapPairs(n1);
		
		 ListNode l = n1;
		 while(l!=null){
			 System.out.println(l.val);
			 l=l.next;
		 }
	}
	public ListNode swapPairs(ListNode h1){
		 ListNode p =h1;
		 
		 while(p!=null && p.next!=null ){
			 //swap
			 int tmp = p.val;
			 p.val = p.next.val;
			 p.next.val = tmp;
			 
			 p=p.next.next;
		 }
         return h1;
         
		
		
	}

}