day6

[0203.移除链表元素]

/**
 * Definition for singly-linked list.
 * 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; }
 * }
 */
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        ListNode dummy = new ListNode (-1, head); 
        ListNode cur = dummy.next;
        while (cur != null){
            if (cur.val == val){
                cur.next = cur.next.next;
                cur = cur.next;
            }
            cur = cur.next;
        }
        return dummy.next;
    }
}

• java.lang.NullPointerException: Cannot read field "next" because ".next" is null，不能读cur.next.next；看来我需要在开始设置一个结点用来记录前一个结点位置

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

• 完成！记得之前本科学数据结构，课上老师经常只讲删除一个元素，而且都是伪代码，课程作业是否写过这种循环删除的印象不深了，所以我对原理的理解没啥大问题，但实际操作有些少，还需要多练练~