java——删除链表中等于给定值的所有元素

class ListNode{
    int val ;
    ListNode next;
    public ListNode(int x) {
        val = x;
    }
    public ListNode(int[] arr) {
        if(arr == null || arr.length == 0) {
            throw new IllegalArgumentException("ARR CAN NOT BE EMPTY");
        }
        
        val = arr[0];
        //这个this是指实现这个内部类的对象
        ListNode cur = this;
        for(int i = 1 ; i < arr.length ; i ++) {
            cur.next = new ListNode(arr[i]);
            cur = cur.next;
        }
    }
    public String toString() {
        StringBuilder res = new StringBuilder();
        ListNode cur = this;
        while(cur != null) {
            res.append(cur.val + "->");
            cur = cur.next;
        }
        res.append("NULL");
        return res.toString();
    }
    

}
public class Main {
    public static void main(String[] args) {
        int[] nums = {1,2,4,3,6,4,2};
        ListNode head = new ListNode(nums);
        System.out.println(head);
        ListNode node = removeElement(head, 2);
        System.out.println(node);
    }
    //不设头节点
    public static ListNode removeElement(ListNode head, int val) {
        //删除开始部分的节点
        while(head != null && head.val == val) {
            head = head.next;
        }
        if(head == null) {
            return head;
        }
        //删除中间部分的节点
        ListNode prev = head;
        while(prev.next != null) {
            if(prev.next.val == val) {
                prev.next = prev.next.next;
            }else {
                prev = prev.next;
            }
        }
        return head;
    } 
}