lintcode174 删除链表中倒数第n个节点

删除链表中倒数第n个节点 

 

给定一个链表，删除链表中倒数第n个节点，返回链表的头节点。

 注意事项

链表中的节点个数大于等于n

您在真实的面试中是否遇到过这个题？ 

Yes

样例

给出链表1->2->3->4->5->null和 n = 2.

删除倒数第二个节点之后，这个链表将变成1->2->3->5->null.

基础：lintcode166 链表倒数第n个节点 

/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */


class Solution {
public:
    /*
     * @param head: The first node of linked list.
     * @param n: An integer
     * @return: The head of linked list.
     */
    ListNode * removeNthFromEnd(ListNode * head, int n) {
        // write your code here
        if (head == NULL || n < 1) return NULL;
        ListNode *first = head;
        ListNode *second = head;
        ListNode *pre;
        while (n) {
            first = first->next;
            n--;
        }
        while (first) {
            first = first->next;
            pre = second;  //标记倒数第n个节点的前一个节点
            second = second->next;
        }
        if (second == head) {
            head = head->next;
        } else {
            pre->next = pre->next->next;
        }

        return head;
    }
};