Remove all elements from a linked list of integers that have value val.

Example
Given: 1 --> 2 --> 6 --> 3 --> 4 --> 5 --> 6, val = 6
Return: 1 --> 2 --> 3 --> 4 --> 5

/**

 * Definition for singly-linked list.

 * struct ListNode {

 *     int val;

 *     ListNode *next;

 *     ListNode(int x) : val(x), next(NULL) {}

 * };

 */

class Solution {

public:

    ListNode* removeElements(ListNode* head, int val) {

        ListNode* p = head,*next=NULL,*pre=NULL;

        while(p && p->val==val){

            next = p->next;

            delete(p);

            p = next;

            head = p;

        }

        while(p){

            if(p->val == val){

                pre->next = p->next;

                delete(p);

                p = pre->next;

            }else{

                pre=p;

                p=p->next;

            }

        }

        return head;

    }

};