leetcode876_鏈表的中間節點

一. 迭代.

1. 時間O(n), 空間O(1).

/**
 * 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) {
        //快慢指針.
        ListNode* slow = head;
        ListNode* fast = head;
        while(fast!=NULL && fast->next!=NULL) {
            slow = slow->next;
            fast = fast->next->next;
        }
        //slow必然是中間節點.
        return slow;
    }
};