Leetcode Remove Nth Node From End of List

Given a linked list, remove the nth node from the end of list and return its head.

For example,

 Given linked list: 1->2->3->4->5, and n = 2.   After removing the second node from the end, the linked list becomes 1->2->3->5.

这道题要考虑单链表是否存在表头节点，题目没说明白导致了提交总是出错，发现此单链表不存在表头 

/** * 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 * p = head;        ListNode *q = head;        int len=0;        while(p)        {            len++;            p = p->next;        }        if(len == 1)            return NULL;        if(n==len)            return head->next;        p = head;        for(int i=0; i<len-n;i++)        {            q = p;            p = p->next;        }        q->next = p->next;        return head;    }};

Submission Result:Accepted