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.

Tag：双指针

struct ListNode{    int val;    ListNode *next;    ListNode(int x) : val(x), next(NULL) {}};ListNode *removeNthFromEnd(ListNode *head, int n){    if(head == NULL)        return NULL;    ListNode *l = head;    ListNode *r = head;    for(int i = 0; i < n; i++)        r = r->next;    if(r == NULL)        return head->next;    while(r->next != NULL)    {        l = l->next;        r = r->next;    }    l->next = l->next->next;    return head;}