876. Middle of the Linked List*

876. Middle of the Linked List*

https://leetcode.com/problems/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.

C++ 实现 1

使用快慢指针. fast 每次走两步, 而 slow 每次走一步, 最终 slow 会指向中间的节点; 另外, 有的时候 slow 和 fast 初始化会决定最后 slow 导致指向哪一个节点, 这个可以用小例子来验证.

class Solution {
public:
    ListNode* middleNode(ListNode* head) {
        if (!head) return nullptr;
        auto slow = head, fast = head;
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        return slow;
    }
};