876. Middle of the Linked List(Leetcode每日一题-2020.03.23)

Problem

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.

Note:

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

Example1

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.

Example2

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.

Solution

经典的快慢指针问题。

/**
 * 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) {
        if(!head)
            return NULL;
        ListNode *fast = head;
        ListNode *slow = head;

        while(fast && fast->next)
        {
            fast = fast->next->next;
            slow = slow->next;
        }

        return slow;
    }
};