876. Middle of the Linked List

1. Description

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.

2. Example

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.

3. Note

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

4. Solutions

My Accepted Solution

n is the number of nodes in i_head
Time complexity: O(n)
Space complexity: O(1)

class Solution {
public:
    ListNode* middleNode(ListNode* i_head) {
        auto slowIter = i_head, fastIter = i_head;
        for (; fastIter && fastIter->next;
             slowIter = slowIter->next, fastIter = fastIter->next->next) {
        }

        return slowIter;
    }
};