203. Remove Linked List Elements

question：

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

answer：

/**
 * 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; 
    ListNode *q;
    p=q=head;
       
    while (q!=NULL)
    {
        if (q->val==val)
        {
            if ( q==head)
            {
                p=head=q->next;
            }
            else
            {
                p->next=q->next;  
            }
           
            q=q->next;
        }
        else
        {
            p=q;
            q=q->next;
        }
    }
   
    return head;
    }
};