一次过，没什么可说的。
ListNode* removeElements(ListNode* head, int val) {
ListNode * tmp = head;
ListNode * pre = NULL;
while(tmp){
if(head == tmp && tmp->val == val){
tmp = tmp->next;
delete head;
head = tmp;
}
else if(tmp->val == val){
pre->next = pre->next->next;
delete tmp;
tmp = pre->next;
}else{
pre = tmp;
tmp = tmp->next;
}
}
return head;
} ListNode* removeElements(L
