反转链表

题目：给定单链表的头节点 head ，请反转链表，并返回反转后的链表的头节点。

定义链表结构：

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;
    }

}

主要思想：

public class reverseList {
    public static void main(String[] args){
        reverseList r = new reverseList();
        ListNode node1 = new ListNode(5);
        ListNode node2 = new ListNode(6,node1);
        ListNode node3 = new ListNode(7,node2);
        r.fun(node3);


    }
    public ListNode fun(ListNode head){
        ListNode preNode = null;
        ListNode curr = head;
        while(curr != null){
            ListNode next = curr.next;
            curr.next = preNode;
            preNode = curr;
            curr = next;
        }
        return preNode;
    }
}