Add problem 143 - Reorder List

Author: anirudhology

problems/linked_list/reorder_list.py

@@ -0,0 +1,46 @@
+from typing import Optional
+
+from problems.util.list_node import ListNode
+
+
+class ReorderList:
+    def reorderList(self, head: Optional[ListNode]) -> ListNode | None:
+        """
+        Do not return anything, modify head in-place instead.
+        """
+        # Special case
+        if head is None or head.next is None:
+            return head
+        # Find the middle of the list
+        middle = self.get_middle(head)
+        # Divide the list in two halves
+        head_1, head_2 = head, middle.next
+        middle.next = None
+        # Find the reverse of the second half
+        reverse_head_2 = self.reverse(head_2)
+        # Merge two lists
+        while head_1 is not None and reverse_head_2 is not None:
+            next_1, next_2 = head_1.next, reverse_head_2.next
+            head_1.next = reverse_head_2
+            reverse_head_2.next = next_1
+            head_1 = next_1
+            reverse_head_2 = next_2
+        return head
+
+    @staticmethod
+    def get_middle(head: Optional[ListNode]) -> ListNode:
+        slow, fast = head, head
+        while fast is not None and fast.next is not None:
+            slow = slow.next
+            fast = fast.next.next
+        return slow
+
+    @staticmethod
+    def reverse(head: Optional[ListNode]) -> ListNode:
+        previous_node, current_node, next_node = None, head, None
+        while current_node is not None:
+            next_node = current_node.next
+            current_node.next = previous_node
+            previous_node = current_node
+            current_node = next_node
+        return previous_node

tests/linked_list/reorder_list_test.py

@@ -0,0 +1,78 @@
+import unittest
+
+from problems.linked_list.reorder_list import ReorderList
+from problems.util.list_node import ListNode
+
+
+def build_list(values):
+    if not values:
+        return None
+    head = ListNode(values[0])
+    current = head
+    for value in values[1:]:
+        current.next = ListNode(value)
+        current = current.next
+    return head
+
+
+def list_to_array(head):
+    result = []
+    current = head
+    while current:
+        result.append(current.val)
+        current = current.next
+    return result
+
+
+class ReorderListTest(unittest.TestCase):
+    def setUp(self):
+        self.reorder_list = ReorderList()
+
+    def test_empty_list(self):
+        head = None
+        self.reorder_list.reorderList(head)
+        self.assertIsNone(head)
+
+    def test_single_element_list(self):
+        head = build_list([1])
+        self.reorder_list.reorderList(head)
+        self.assertEqual(list_to_array(head), [1])
+
+    def test_two_element_list(self):
+        head = build_list([1, 2])
+        self.reorder_list.reorderList(head)
+        self.assertEqual(list_to_array(head), [1, 2])
+
+    def test_three_element_list(self):
+        head = build_list([1, 2, 3])
+        self.reorder_list.reorderList(head)
+        self.assertEqual(list_to_array(head), [1, 3, 2])
+
+    def test_four_element_list(self):
+        head = build_list([1, 2, 3, 4])
+        self.reorder_list.reorderList(head)
+        self.assertEqual(list_to_array(head), [1, 4, 2, 3])
+
+    def test_odd_number_of_elements(self):
+        head = build_list([1, 2, 3, 4, 5])
+        self.reorder_list.reorderList(head)
+        self.assertEqual(list_to_array(head), [1, 5, 2, 4, 3])
+
+    def test_even_number_of_elements(self):
+        head = build_list([1, 2, 3, 4, 5, 6])
+        self.reorder_list.reorderList(head)
+        self.assertEqual(list_to_array(head), [1, 6, 2, 5, 3, 4])
+
+    def test_get_middle(self):
+        head = build_list([1, 2, 3, 4, 5])
+        middle = self.reorder_list.get_middle(head)
+        self.assertEqual(middle.val, 3)
+
+    def test_reverse_list(self):
+        head = build_list([1, 2, 3, 4, 5])
+        reversed_head = self.reorder_list.reverse(head)
+        self.assertEqual(list_to_array(reversed_head), [5, 4, 3, 2, 1])
+
+
+if __name__ == '__main__':
+    unittest.main()