LeetCode Day17 Remove-the-Nth-node-from-end-of-list

一前一后两指针，让前面的先走，当两个指针距离超过n，后面的往前移

class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        if(!head->next) return NULL;
        ListNode *hb=head,*hf=head;
        //for(int i=0;i<n;i++) hf=hf->next;
        int flag=0;
        while(hf->next){
            hf=hf->next;
            flag++;
            if(flag>n){
                hb=hb->next;
                flag--;
            }
        }
       if(flag<n) return hb->next;
        //cout<<hb->val<<"and"<<hf->val<<endl;
        hb->next=hb->next->next;
        return head;
        
    }
};

一前一后两指针，让前面的先走n步，然后前后一起移动

class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        if(head->next==NULL) return NULL;
        ListNode *pb = head, *pf = head;
        for(int i = 0; i < n; ++i) pf = pf->next;
        if(!pf) return head->next;
        while(pf->next){
            pb = pb->next;
            pf = pf->next;
        }

        pb->next = pb->next->next;

        return head;

    }

};