牛客网 BM9 删除链表的倒数第n个节点

class Solution {
public:
    

    ListNode* removeNthFromEnd(ListNode* head, int n) {
        // write code here
        if (head == NULL)
            return NULL;
        ListNode* res = new ListNode(0);
        res->next = head;
        
        //当前节点
        ListNode* cur = head;
        //前节点
        ListNode* pre = res;
        ListNode* fast = head;
        //先让fast快走N步；
        while (n--)
            fast = fast->next;
        //快慢指针同步，快指针到底最后，慢指针正好到了倒数第N个位置
        while (fast != NULL) {
            fast = fast->next;
            pre = cur;
            cur = cur->next;
        }
        //删除该位置的节点
        pre->next = cur->next;
        //返回头节点
        return res->next;
    }
};