Add sum_linked_lists with doctests

data_structures/linked_list/sum_linked_lists.py

@@ -0,0 +1,102 @@
+"""
+Given two numbers represented by linked lists, where each node
+contains a single digit, write a function that adds the two
+numbers and returns the sum as a linked list. The digits are
+stored in reverse order, meaning the 1's digit is at the
+head of the list.
+
+Explanation from GeeksForGeeks: https://www.geeksforgeeks.org/dsa/add-number-and-its-reverse-represented-by-linked-list/
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass
+class ListNode:
+    value: int = 0
+    next_node: ListNode | None = None
+
+
+def create_linked_list(values: list[int]) -> ListNode | None:
+    """
+    Helper function to create a linked list from a list of values.
+
+    >>> head = create_linked_list([2, 4, 3])
+    >>> (head.value, head.next_node.value, head.next_node.next_node.value)
+    (2, 4, 3)
+    >>> create_linked_list([]) is None
+    True
+    """
+    if not values:
+        return None
+    head = ListNode(values[0])
+    current = head
+    for value in values[1:]:
+        current.next_node = ListNode(value)
+        current = current.next_node
+    return head
+
+
+def linked_list_to_list(head: ListNode | None) -> list[int]:
+    """
+    Helper function to convert a linked list to a list of values.
+
+    >>> head = ListNode(2)
+    >>> head.next_node = ListNode(4)
+    >>> head.next_node.next_node = ListNode(3)
+    >>> linked_list_to_list(head)
+    [2, 4, 3]
+    >>> linked_list_to_list(None)
+    []
+    """
+    values = []
+    current = head
+    while current:
+        values.append(current.value)
+        current = current.next_node
+    return values
+
+
+def sum_linked_lists(l1: ListNode | None, l2: ListNode | None) -> ListNode | None:
+    """
+    Adds two numbers represented by linked lists and returns the sum as a linked list.
+
+    Args:
+        l1: The head of the first linked list.
+        l2: The head of the second linked list.
+    Returns:
+        The head of the linked list representing the sum of the two numbers.
+    >>> l1 = create_linked_list([2, 4, 3])  # Represents the number 342
+    >>> l2 = create_linked_list([5, 6, 4])  # Represents the number 465
+    >>> linked_list_to_list(sum_linked_lists(l1, l2))  # Represents the number 807
+    [7, 0, 8]
+    >>> l1 = create_linked_list([0])  # Represents the number 0
+    >>> l2 = create_linked_list([0])  # Represents the number 0
+    >>> linked_list_to_list(sum_linked_lists(l1, l2))  # Represents the number 0
+    [0]
+    >>> l1 = create_linked_list([9, 9, 9, 9, 9, 9, 9])  # Represents the number 9999999
+    >>> l2 = create_linked_list([9, 9, 9, 9])  # Represents the number 9999
+    >>> linked_list_to_list(sum_linked_lists(l1, l2))  # Represents the number 10009998
+    [8, 9, 9, 9, 0, 0, 0, 1]
+    """
+    dummy_head = ListNode(0)
+    current = dummy_head
+    carry = 0
+
+    while l1 or l2 or carry:
+        val1 = l1.value if l1 else 0
+        val2 = l2.value if l2 else 0
+
+        total = val1 + val2 + carry
+        carry = total // 10
+        current.next_node = ListNode(total % 10)
+        current = current.next_node
+
+        if l1:
+            l1 = l1.next_node
+        if l2:
+            l2 = l2.next_node
+
+    return dummy_head.next_node