链表

链表结构

public class ListNode {
    int val;
    ListNode next;
    public ListNode() {
    }
    public ListNode(int val) {
        this.val = val;
    }
    public ListNode(int val, ListNode next) {
        this.val = val;
        this.next = next;
    }
}

添加虚拟头节点

看到删除的题目，考虑虚拟头节点
添加虚拟节点后，删除操作可以统一

public ListNode removeElements(ListNode head, int val) {
        if (head == null) return null;
        ListNode dummy = new ListNode(-1, head);
        ListNode pre = dummy;
        ListNode cur = head;
        while (cur != null) {
            if (cur.val == val) {
                pre.next = cur.next;
            }
            //不相等，则移动pre指针
            else {
                pre = cur;
            }
            cur = cur.next;
        }
        return dummy.next;
    }public ListNode removeElements(ListNode head, int val) {
    if
    }

设计链表

707. 设计链表

反转链表

双指针

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