題目要求 （高頻題）

給一個帶有頭節點head並且非空的單鏈表。返回連結串列的中間節點。
如果有兩個中間結點，那麼返回第二個。

示例

// Example 1
Input: [1,2,3,4,5]
Output: Node 3 from this list (Serialization: [3,4,5])
The returned node has value 3.  (The judge's serialization of this node is [3,4,5]).
Note that we returned a ListNode object ans, such that:
 

ans.val = 3, ans.next.val = 4, ans.next.next.val = 5, and ans.next.next.next = NULL.

//Example 2
Input: [1,2,3,4,5,6]
Output: Node 4 from this list (Serialization: [4,5,6])
Since the list has two middle nodes with values 3 and 4, we return the second one.

Note: the number of nodes in the given list will be between 1
 
 and 100.

解體思路

快慢指標法

和 leetcode 141 判斷連結串列中是否有環 的思路是一樣的，通過設定快慢指標，從表頭開始遍歷，快指標每次走兩步，慢指標走一步。

當快指標到達連結串列尾時。慢指標剛好到達連結串列中點，返回慢指標即可

/**
 * 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* slow = head;
        ListNode* fast = head;
        while (fast && fast->next) //判斷非空條件
        {
            slow = slow->next;
            fast = fast->next->next;
        }
        return slow;
    }
};

原題連結： https://leetcode.com/problems/middle-of-the-linked-list/submissions/