203.移除链表元素

输入： head = [1,2,6,3,4,5,6], val = 6 输出： [1,2,3,4,5]

class Solution {
    public ListNode removeElements(ListNode head, int val) {
        if(head == null){
            return head;
        }
        ListNode dummy = new ListNode(-1,head);
        ListNode pre = dummy;
        ListNode cur = head;
        while(cur != null){
            if(cur.val == val){
                pre.next = cur.next;
            }else{
                pre = cur;
            }
            cur = cur.next;
        }
        return dummy.next;
    }
}

206.反转链表

输入： head = [1,2,3,4,5]
输出： [5,4,3,2,1]

class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode pre = null;
        ListNode temp = null;
        ListNode cur = head;
        while(cur != null){
            temp = cur.next;
            cur.next = pre;
            pre = cur;
            cur = temp;
        }
        return pre;
    }
}