LeetCode: 876. Middle of the Linked List

题目描述

Given a non-empty, singly linked list with head node head, return a middle node of linked list.

If there are two middle nodes, return the second middle node.

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.

解题思路

签到题。第一次遍历统计 链表长度。第二次遍历到中间节点返回。

AC代码

/** * 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) {
        int cnt = 0;
        ListNode* iterNode = head;
        while(iterNode != nullptr)
        {
            ++cnt;
            iterNode = iterNode->next;
        }

        iterNode = head;
        cnt = cnt/2;
        while(iterNode && cnt--)
        {
            iterNode = iterNode->next;
        }

        return iterNode;
    }
};