LC206.java

class Solution {
    // 头插法
    public ListNode reverseList(ListNode head) {
        ListNode dummy = new ListNode(-1);
        dummy.next = head;

        ListNode pre = dummy;
        ListNode cur = head;

        while (cur!=null && cur.next!=null) {
            ListNode newHead = cur.next;
            cur.next = cur.next.next;
            newHead.next = pre.next;
            pre.next = newHead;
        }

        return dummy.next;
    }
    
    // 
    public ListNode reverseList2(ListNode head) {
            ListNode pre = null;
        ListNode cur = head;
        ListNode tmp = null;
        while (cur != null) {
            tmp = cur.next;
            cur.next = pre;
            pre = cur;
            cur = tmp;
        }

        return pre;
    
    }
}