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;
}
};