linked list sum lists

linked_list_sum_lists_2_5.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

# Question.
# You have two numbers represented by a linked list, where each node contains a single
# digit. The digigts are stored in reverse order, such that the 1's digit is at the head of the list.
# Write a function that adds the two numbers and returns the sum as a linked list.
# Example
# Input: (7 -> 1 -> 6) + (5 -> 9 -> 2). That is, 617 + 295
# Output: 2 -> 1 -> 9. That is, 912.
# Follow Up
# Suppose the digits are stored in forward order. Repeat the above  problem.
# Example
# Input: (6 -> 1 -> 7) + (2 -> 9 -> 5). That is, 617+295
# Output: 9 -> 1 -> 2. That is, 912

class Node:
  def __init__(self, value, next=None):
    self.value = value
    self.next_ptr = next

  def __str__(self):
    return "%d" % (self.value)

class LinkedList:
  def __init__(self):
    self.edge_node = None
    self.head_node = None

  def add_node(self, node):
    if not self.edge_node:
      self.head_node = node
    node.next_ptr = self.edge_node
    self.edge_node = node

  def add_node_to_head(self, node):
    if self.head_node:
      self.head_node.next_ptr = node
    if not self.edge_node:
      self.edge_node = node
    self.head_node = node

  def add(self, node_value):
    new_node = Node(node_value, self.edge_node)
    self.add_node(new_node)

    return new_node

  def add_to_head(self, node_value):
    new_node = Node(node_value)
    self.add_node_to_head(new_node)

  def __add__(self, ll):
    result_ll = LinkedList()

    cur_n_1, cur_n_2 = self.edge_node, ll.edge_node
    cur_sum = cur_n_1.value + cur_n_2.value
    while cur_sum > 0:
      modulus = cur_sum % 10
      result_ll.add_to_head(modulus)

      cur_n_1 = cur_n_1.next_ptr if (cur_n_1 and cur_n_1.next_ptr) else None
      cur_n_2 = cur_n_2.next_ptr if (cur_n_2 and cur_n_2.next_ptr) else None

      cur_sum = cur_sum / 10 # quotient
      cur_sum += cur_n_1.value if cur_n_1 else 0
      cur_sum += cur_n_2.value if cur_n_2 else 0

    return result_ll

  def __str__(self):
    node_value_arr = []
    node = self.edge_node
    while node:
      node_value_arr.append(str(node))
      node = node.next_ptr

    return " -> ".join(node_value_arr)

if __name__ == '__main__':
  # Setup linkedlist
  linked_list_1 = LinkedList()
  # linked_list_1.add(9)
  linked_list_1.add(6)
  linked_list_1.add(1)
  linked_list_1.add(7)

  linked_list_2 = LinkedList()
  linked_list_2.add(2)
  linked_list_2.add(9)
  linked_list_2.add(5)

  print "(%s) + (%s) = (%s)" % (linked_list_1, linked_list_2, (linked_list_1 + linked_list_2))