LeeCode 刪除單鏈表中的某一特定值元素

題目：

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

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

C代碼：

/*struct ListNode
{
     int val;
     struct ListNode *next;
};
*/ 
struct ListNode* removeElements(struct ListNode* head, int val) {
    if(NULL == head) return head;

    struct ListNode *p=head; 
    struct ListNode *pre=NULL; 

    while(NULL != p){
        if(p->val == val){
            struct ListNode *del =p;
            p=p->next;
            if(NULL!= pre){  
                pre->next=p;
            }else{
                head=p;
            }
            free(del);
        } else{
            pre=p;
            p=p->next;
        }
    }

    return head;
}