要點：1 head正向走動k個  2 另外一個pre、head1和head 走，至到head盡頭 3 刪除

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *removeNthFromEnd(ListNode *head, int n) {
        if(head==NULL||n<=0){
            return NULL;
        }
        ListNode *h1=head,*h2=head;;
        ListNode *pre=NULL;
        for(int i=0;i<n;i++){
            if(h1){
                h1=h1->next;
            }else{
                return NULL;
            }
        }
        
        while(h1){
            h1=h1->next;
            pre=h2;
            h2=h2->next;
        }
        
        if(pre==NULL){
           head=head->next;
           delete h2;
           return head;
        }
        pre->next=h2->next;
        delete h2;
        h2=NULL;
        return head;
        
    }
};



        if(head==NULL||n<=0){
            return head;
        }
        ListNode *temp=head;
        ListNode *slow=head;
        ListNode *pre=NULL;
        for(int i=0;i<n;++i){
            if(temp){
                temp=temp->next;
            }else{
                return NULL;
            }
        }
        
        while(temp){
            temp=temp->next;
            slow=slow->next;
            pre=slow;
        }
        
        if(pre==NULL){
            head=head->next;
            return head;
        }
        
        pre->next=slow->next;
        slow=NULL;
        
        return head;
        
    }