[LeetCode]Middle of the Linked List

双指针的问题，两个指针一快一慢，快的每次移动2的距离，慢的移动1的距离即可。时间复杂度O(N)，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* middleNode(ListNode* head) {
	ListNode* fast = head, *slow = head;
	while(fast && fast->next)
	{
	fast = fast->next->next;
	slow = slow->next;
	}
	return slow;
	}
	};

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