# LeetCode #2: Add Two Numbers

·520 words·3 mins
coding leetcode python

## Description #

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.


(From the Add Two Numbers problem description)

## Solutions #

### Problem API #

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:


class ListNode:
def __init__(self, x):
self.val = x
self.next = None

def __str__(self):
if self.next:
return f"{self.val}{self.next}"
else:
return f"{self.val}"


#### Tests #

The problem description can be validated by the following tests:

import os
import pytest

from supplemental import ListNode

# --- helper methods ---
def from_py(*digits):
digits = list(digits)
while digits:
n.next = ListNode(digits.pop(0))
n = n.next

r = []
return r

def test_transformer():
""" Make sure that the test tooling works. """
for test_value in [ , [1,2], [1,2,3] ]:

# --- END helper methods ---

class BaseTestClass:
def test_vector_from_spec_ok(self):
a = from_py(2,4,3)
b = from_py(5,6,4)

a = from_py(0)
b = from_py(0)

a = from_py(5)
b = from_py(7)

def test_different_lengths(self):
a = from_py(1,8)
b = from_py(0)



### Solution 1: iterative $$O(n)$$ #

The simplest solution probably is to just iterate over both lists and add the digits. The only “edge” case is that the carry needs to be taken of.

Time complexity
$$O(n)$$.
Space complexity
$$O(n)$$.

from supplemental import ListNode

class Solution:
def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
carry = 0
tail = None
while l1 or l2:
v1 = l1.val if l1 else 0
v2 = l2.val if l2 else 0

sum = v1 + v2 + carry
digit = sum % 10
carry = 1 if sum >= 10 else 0

else:
tail.next = ListNode(digit)
tail = tail.next

if l1: l1 = l1.next
if l2: l2 = l2.next
if carry:
tail.next = ListNode(carry)


#### Test Results #

With the following test result:

============================= test session starts ==============================
platform linux -- Python 3.7.5, pytest-3.10.1, py-1.8.0, pluggy-0.12.0
benchmark: 3.2.2 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000)
plugins: benchmark-3.2.2, profiling-1.7.0
collected 4 items

test_1a-solution.py ....                                                 [100%]

============================ slowest test durations ============================

(0.00 durations hidden.  Use -vv to show these durations.)
=========================== 4 passed in 0.03 seconds ===========================

Tests ran in 0.17746263899607584 seconds


## Summary #

Solution Correct Status Runtime Memory Language
#1, iterative Yes Accepted 64 ms, faster than 93,77% 12,6 MB, less than 100% Python3