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.

Note:
Given n will always be valid.
Try to do this in one pass.

Solutions:

/** * 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(n<1 || head == NULL)            return head;        ListNode* pre = head;        int i=0;        while(i<n && pre != NULL)        {            pre = pre->next;            i++;        }        if(i<n)            return head;        if(pre == NULL)            return head->next;        ListNode* back = head;        ListNode* save;        while(pre != NULL)        {            pre = pre->next;            save = back;            back = back->next;        }        save->next = back->next;        delete back;        return head;    }};