leetcode19——Remove Nth Node From End of List

题目大意：使用一趟扫描删除链表的倒数第n个结点

分析：链表的考察

代码：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode* q = head;
        while(n--){
            q = q->next;
        }
        if(!q) return head->next;  //倒数第n个是正数第一个
        ListNode* p = head;
        while(q->next){
            p = p->next;
            q = q->next;
        }
        p->next = p->next->next;
        return head;
    }
};