1 Remove Linked List Elements

2 测试自己的代码

package linked_list_leetcode;


class ListNode {
    int val;
    ListNode next;

    ListNode(int x) {
        val = x;
    }

    public ListNode(int[] arr) {
        if (arr == null || arr.length == 0) {
            throw new IllegalArgumentException("arr can not be empty!");
        }

        this.val = arr[0];

        ListNode cur = this;

        for (int i = 1; i < arr.length; i++) {
            cur.next = new ListNode(arr[i]);
            cur = cur.next;
        }
    }

    @Override
    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 RemoveElements_203 {
    public ListNode removeElements(ListNode head, int val) {

        while (head != null && head.val == val) {
            ListNode delNode = head;
            head = head.next;
            delNode.next = null;
        }

        if (head == null) {
            return null;
        }

        ListNode prev = head;
        while (prev.next != null) {
            if (prev.next.val == val) {
                ListNode delNode = prev.next;
                prev.next = delNode.next;
                delNode = null;
            } else {
                prev = prev.next;
            }
        }

        return head;
    }


    public ListNode removeElements2(ListNode head, int val) {

        while (head != null && head.val == val) {
            head = head.next;
        }

        if (head == null) {
            return null;
        }

        ListNode prev = head;
        while (prev.next != null) {
            if (prev.next.val == val) {
                prev.next = prev.next.next;
            } else {
                prev = prev.next;
            }
        }

        return head;
    }

    /*
     *
     * 使用虚拟头结点
     * */
    public static ListNode removeElements3(ListNode head, int val) {

        ListNode dummyHead = new ListNode(-1);
        dummyHead.next = head;

        ListNode prev = dummyHead;

        while (prev.next != null) {
            if (prev.next.val == val) {
                prev.next = prev.next.next;
            } else {
                prev = prev.next;
            }
        }

        return dummyHead.next;
    }

    public static void main(String[] args) {
        int[] nums = {1, 2, 6, 3, 4, 5, 6};

        ListNode head = new ListNode(nums);
        System.out.println(head);

        removeElements3(head, 6);
        ListNode res = removeElements3(head,6);
        System.out.println(res);
    }
}