反转链表

class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode prev = null;
        ListNode curr = head;
        while(curr != null) {
            ListNode nxt = curr.next;
            curr.next = prev; // 翻转箭头
            prev = curr; //三人行
            curr = nxt; //三人行
        }
        return prev;
    }
}