[Linked List]Remove Nth Node From End of List

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 /*
 求长度
 找前驱
 删除
 */
class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        int len = 0;
        ListNode* p = head;
        while(p){
            len++;
            p=p->next;
        }
        p = head;
        ListNode* pre = NULL;
        int m = len -n;
        while(m){
            pre = p;
            p=p->next;
            m--;
        }
        if(pre){
            pre->next = p->next;
        }else{
            p = head;
            head = head->next;
        }
        delete(p);
        return head;
    }
};