Create: linked_list_middle.go to solve https://leetcode.com/problems/middle-of-the-linked-list/description/

Author: galfudim

Linked List/linked_list_middle.go

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+/**
+ * Given the head of a singly linked list, return the middle node of the linked list. If there are two middle nodes, return the second middle node.
+ *
+ * Example #1:
+ * Input: head = [1,2,3,4,5]
+ * Output: [3,4,5]
+ * Explanation: The middle node of the list is node 3.
+ * 
+ * Example #2:
+ * Input: head = [1,2,3,4,5,6]
+ * Output: [4,5,6]
+ * Explanation: Since the list has two middle nodes with values 3 and 4, we return the second one.
+ *
+ * Constraints: 
+ * The number of nodes in the list is in the range [1, 100].
+ * 1 <= Node.val <= 100
+ * 
+ * Definition for singly-linked list.
+ * type ListNode struct {
+ *     Val int
+ *     Next *ListNode
+ * }
+ *
+ * Solution: to find the middle of a linked list, we can use two pointers ("slow" and "fast"). They should both start at head. 
+ * While traversing the list, we move the slow pointer one node at a time and the fast pointer two nodes at a time. 
+ * When the fast pointer reaches the end of the linked list, the slow pointer will be pointing to the middle node.
+ *
+ * Time complexity: O(n), space complexity: O(n)
+ */
+ func middleNode(head *ListNode) *ListNode {
+    slow, fast := head, head
+    
+    for (fast != nil && fast.Next != nil) {
+        slow = slow.Next
+        fast = fast.Next.Next
+    }
+
+    return slow
+}