Add 'Middle of the Linked List'

Author: nickolashkraus

README.md

@@ -32,6 +32,8 @@ A collection of LeetCode solutions
 
 [Merge Two Sorted Lists](./src/merge_two_sorted_lists.py)
 
+[Middle of the Linked List](./src/middle_of_the_linked_list.py)
+
 [Product of Array Except Self](./src/product_of_array_except_self.py)
 
 [Remove Nth Node From End of List](./src/remove_nth_node_from_end_of_list.py)

src/middle_of_the_linked_list.py

@@ -0,0 +1,68 @@
+"""
+876. Middle of the Linked List
+
+https://leetcode.com/problems/middle-of-the-linked-list
+
+NOTES
+  * Use two pointers. Advance the `fast` pointer by 2. Advance the `slow`
+    pointer by 1. When `fast` reaches the end of the linked list, `slow` is at
+    the middle node.
+
+    Example (where ○ represents null):
+
+      1   2   3   4   5
+      ● → ● → ● → ● → ● → ○  middle = 3
+      ↑
+      slow
+      fast
+
+      1   2   3   4   5
+      ● → ● → ● → ● → ● → ○  middle = 3
+          ↑   ↑
+          slow
+              fast
+
+      1   2   3   4   5
+      ● → ● → ● → ● → ● → ○  middle = 3
+              ↑       ↑
+              slow
+                      fast
+
+    When the list contains an even number of nodes, there are two middle nodes.
+    To take the second middle node, use slow.next:
+
+      1   2   3   4   5   6
+      ● → ● → ● → ● → ● → ● → ○  middle = 4
+      ↑
+      slow
+      fast
+
+      1   2   3   4   5   6
+      ● → ● → ● → ● → ● → ● → ○  middle = 4
+          ↑   ↑
+          slow
+              fast
+
+      1   2   3   4   5   6
+      ● → ● → ● → ● → ● → ● → ○  middle = 4
+              ↑       ↑
+              slow
+                      fast
+"""
+
+from src.classes import ListNode
+
+
+class Solution:
+    def middleNode(self, head: ListNode | None) -> ListNode | None:
+        if not head:
+            return head
+
+        slow, fast = head, head
+
+        while fast.next and fast.next.next:
+            slow = slow.next
+            fast = fast.next.next
+
+        # Handle odd and even lists.
+        return slow.next if fast.next else slow

tests/test_middle_of_the_linked_list.py

@@ -0,0 +1,25 @@
+"""
+876. Middle of the Linked List
+
+https://leetcode.com/problems/middle-of-the-linked-list
+"""
+
+from unittest import TestCase
+
+from src.middle_of_the_linked_list import Solution
+
+from .utils import create_linked_list_from_list, create_list_from_linked_list
+
+
+class TestSolution(TestCase):
+    def test_1(self):
+        exp = [3, 4, 5]
+        l1 = create_linked_list_from_list([1, 2, 3, 4, 5])
+        ll = Solution().middleNode(l1)
+        assert create_list_from_linked_list(ll) == exp
+
+    def test_2(self):
+        exp = [4, 5, 6]
+        l1 = create_linked_list_from_list([1, 2, 3, 4, 5, 6])
+        ll = Solution().middleNode(l1)
+        assert create_list_from_linked_list(ll) == exp