[LeetCode]Remove Nth Node From End of List

双指针找到倒数第N + 1个节点，然后通过这个这节点删除倒数第N个节点。注意head有可能会被删除，我们需要用一个helper节点来帮我们keep track of新的head。O(N)时间，常数空间。代码如下：

	/**
	* 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* helper = new ListNode(-1);
	helper->next = head;
	auto fast = head, slow = helper;
	while(n--)fast = fast->next;
	while(fast)
	{
	fast = fast->next;
	slow = slow->next;
	}
	auto tmp = slow->next;
	slow->next = tmp->next;
	tmp->next = nullptr;
	return helper->next;
	}
	};

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