Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

样例

Given 1->2->3->4, you should return the list as 2->1->4->3.

挑战

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    /**
     * @param head a ListNode
     * @return a ListNode
     */
    public ListNode swapPairs(ListNode head) {
        // Write your code here
        ListNode tempN=head;
        int temp=0;
        while(head!=null && head.next!=null)
        {
            temp=head.next.val;
            head.next.val=head.val;
            head.val=temp;
            if(head.next.next!=null)
            {
                head=head.next.next;
            }
            else
            {
                return tempN;
            }
            
        }
        return tempN;
    }
}