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.

class Solution {public:ListNode *removeNthFromEnd(ListNode *head, int n) {map<int,ListNode*> mp;if(head == NULL || head->next == NULL)return NULL;int cnt = 1;while(head != NULL){mp[cnt++] = head;head = head->next;}cnt--;if(cnt - n + 1 == 1){delete head;head = mp[2];}else{mp[cnt - n]->next = mp[cnt - n + 1]->next;delete mp[cnt - n + 1];head = mp[1];}return head;}};