LeetCode19 Remove Nth Node From End of List

题意：

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.

Try to do this in one pass. (Easy)

分析：题目虽然是easy，但是用one pass做起来还是包含几个链表常用的手法的，有参考价值；

链表算法上没有什么难的，就是代码上仔细，再熟悉几种处理方法即可。

1.Two pointers。 利用快慢指针，快指针先走n步，然后一起走，快的下一步到终点了，慢的到达要删除的前一位；

2. dummy node。 凡是head位置可能被删掉，返回出现问题，用dummy node处理返回问题。

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 dummy(0);
        dummy.next = head;
        head = &dummy;
        ListNode* chaser = head;
        ListNode* runner = head;
        for (int i = 0; i < n; ++i) {
            runner = runner -> next;
        }
        while (runner -> next != nullptr) {
            runner = runner -> next;
            chaser = chaser -> next;
        }
        ListNode* temp = chaser -> next;
        chaser -> next = chaser -> next -> next;
        delete temp;
        return dummy.next;
    }
};