【LeetCode OJ】Remove Nth Node From End of List

题目：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.

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
    
};
ListNode *removeNthFromEnd(ListNode *head, int n)
{
    ListNode *r = head, *s;
    if (head == NULL)
        return head;
    int i = 0;
    for (ListNode *p = head; p != NULL; p = p->next)
    {
        i++; 
    }
    if (i == n)  //删除第一个节点
    {
        ListNode *l = head->next;
        free(head);
        return l;
    }
    for (int num = 0; num < i - n - 1; num++)//找到要删节点的前一个节点
    {
        r = r->next;
    }
    ListNode *tmp = r->next;
    r->next = r->next->next;
    free(tmp);
    return head;
    
}