leetcode 19. Remove Nth Node From End of List 单向链表删除第n个数

代码：

/**
* 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) {
ListNode *now = head;
ListNode *tmp = head;
int len=0;
while(tmp) {
len++;
tmp=tmp->next;
}
n=len-n;
if(head->next==NULL) return now=NULL;
if(n==0){
return head->next;
}
int num=0;
while(head){
if(num+1==n) {
if(head->next){
ListNode *tmp = head->next;
head->next=tmp->next;
head=head->next;
tmp=NULL;
} else {
head->next=NULL;
head=head->next;
break;
}
}
else head=head->next;
num++;
}
//cout<<now->val<<endl;
return now;
}
};