fast 先走N步
slow fast然后一起走

D->1->2->3->4->5 n=2 remove 4 

1, fast move 2 stesp to node "2"
while(n--){
    fast = fast->next;
}
2, fast slow move togeter
while(fast->next){
    fast = fast->next;
    slow = slow->next;
}
when fast == "5" slow = "3"

3, slow->next = slow->next->next;

struct ListNode * removeNthFromEnd(struct ListNode* head, int n){
    if(head == NULL)
        return NULL;
    struct ListNode * fast, *slow, *tmp, *tmp2;
    struct ListNode *dummy = calloc(1, sizeof(struct ListNode));
    dummy->next = head;
    fast = slow = dummy;
    
    while(n--){
        fast = fast->next;
    }
    while(fast->next){
        fast = fast->next;
        slow = slow->next;
    }
    tmp = slow->next;   
    slow->next = slow->next->next;
    free(tmp);
    tmp2 = dummy->next;
    free(dummy);
    return tmp2;

}