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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

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* removeElements(struct ListNode* head, int val)
{
    struct ListNode* pre, *p;
    pre = p = head;
       
    while ( p )
    {
        if ( p->val == val)
        {
            if ( p == head)
            {
                pre = head = p->next;
            }
            else
            {
                pre->next = p->next;  
            }
           
            free(p);
            p = p->next;
        }
        else
        {
            pre = p;
            p = p->next;
        }
    }
   
    return head;
}