题解 | #反转链表# 2种解法：双指针、递归

双指针

public class Solution {
    public ListNode ReverseList(ListNode head) {
        if (head == null) {
            return null;
        } else {
            ListNode next = null;
            ListNode pre = null;

            while (head != null) {
                next = head.next;
                head.next = pre;
                pre = head;
                head = next;
            }
            return pre;
        }
    }
}

递归

public class Solution {
    public ListNode ReverseList(ListNode head) {
        if (head == null || head.next == null) 
            return head;
        ListNode last = ReverseList(head.next);
        head.next.next = head;
        head.next = null;
        return last;
    }
}