/**
 * 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) {

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

        if(head == null)
            return head;

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

/**
 * 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 Solution2 {
    public ListNode removeElements(ListNode head, int val) {

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


        if(head == null)
            return head;

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

class Solution3 {
    public ListNode removeElements(ListNode head, int val) {

        if(head == null) //最小规模情况结束条件
            return head;

        head.next = removeElements(head.next, val);
        return head.val == val ? head.next : head; 

    }