leetCode 24. Swap Nodes in Pairs (双数交换节点) 解题思路和方法

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.

思路：题目比較简单，会链表反转的都能够做，思路也差点儿相同。不多说。上代码。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
	    public ListNode swapPairs(ListNode head) {
	        
	        ListNode firstHead = new ListNode(0);
	        firstHead.next = head;
	        ListNode pre = firstHead;//定义一个头结点，这样全部的操作都同样
	        ListNode next = null;
	        while(head != null && head.next != null){
	        	//A-B-C-D交换BC,pre=A;B=head;C=next
	            next = head.next;//保存交换的变量C
	            
	            head.next = next.next;//将B指向B的指针指向D
	            pre.next = next;//将A指向B的指针指向C
	            next.next = head;//将C指向D的指针指向B,完毕交换，顺序变为A-C-B-D
	            
	            //为下一循环准备变量
	            pre = head;//将pre变为B
	            head = head.next;//将head指向D
	            
	        }
	        return firstHead.next;
	    }
}