反转链表

题：输入一个链表，反转链表后，输出新链表的表头。

方法一：递归

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head == null || head.next == null) {
             return head;
         }
         ListNode res = ReverseList(head.next);
         head.next.next = head;
         head.next = null;
         return res;
    }
}

方法二：迭代

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode ReverseList(ListNode head) {
        ListNode res = null;
        while(head != null){
            ListNode temp = head.next;
            head.next = res;
            res = head;
            head = temp;
        }
        return res;
    }
}