剑指15.反转链表

/*
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 pre = null;//注意要考虑到第一个节点，让它指向null
        ListNode next = null;
        while(head != null){
            next = head.next; //先记录一下当前节点的下一个节点，以防断开找不到下一个节点
            head.next = pre;
            pre = head;
            head = next;
        }
        return pre;
    }
}