反转链表

/**
* 翻转链表非递归
*/
public ListNode q1(ListNode head){
ListNode left =null;
ListNode right =head;
ListNode right2=head;

while(right!=null){
right2 = right.next;
right.next = left;
left = right;
right = right2;

}

return left;

}

/**
* 递归 先深入到底，再逆方向操作
*/

public ListNode q2(ListNode head){

if(headnull || head.nextnull){
return head;
}
ListNode newHead = q2(head.next);
head.next.next = head;
head.next = null;

return newHead;
}

public class ListNode {
int val;
ListNode next;
ListNode() {}
ListNode(int val) { this.val = val; }
ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}