19. Remove Nth Node From End of List 链表遍历

题目地址

设置两个指针，一个先走n步，然后两个同步走，这样当前一个走到表尾的时候，后一个就走到了倒数第n个。

/** * 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 *p = head, *q = head, *pre;        if (head == NULL || head->next == NULL)            return NULL;        while (n--) p = p->next;        if (p == NULL) {            pre = head;            head = head->next;            delete pre;            return head;        }        while (p->next != NULL) {            p = p->next;            q = q->next;        }        pre = q->next;        q->next = pre->next;        delete pre;        return head;    }};