[LeetCode]Swap Nodes in Pairs 成对交换

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：肯定是递归方便啦，我们做好了base case，一直调用自己就能够了。 要是想不出来base case是什么，多理解理解就熟悉了

public ListNode swapPairs(ListNode head) {
       if(head==null) return null;
       if(head.next==null) return head;
       ListNode temp=head.next;
       ListNode forward=head.next.next;
       temp.next=head;
       head.next=swapPairs(forward);
       return temp;
   }