翻转链表

链表的结构：

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;
    }
}

翻转链表的算法：

public ListNode reverseNode(ListNode l) {
    if (l == null || l.next == null)
        return l;
    ListNode p = l, pre = null, next = null;
    while (p.next != null) {
        next = p.next;
        p.next = pre;
        pre = p;
        p = next;
    }
    p.next = pre;
    return p;
}