Leetcode #445: Add Two Numbers II
In this guide, we solve Leetcode #445 Add Two Numbers II in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contains a single digit.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Stack, Linked List, Math
Intuition
The problem has a natural nested or last-in-first-out structure.
A stack lets us resolve matches in the correct order as we scan.
Approach
Push items as they appear and pop when you can finalize a decision.
The stack captures the unresolved part of the input.
Steps:
- Push elements as you scan.
- Pop when a rule or match is satisfied.
- Use the stack to compute results.
Example
Input: l1 = [7,2,4,3], l2 = [5,6,4]
Output: [7,8,0,7]
Python Solution
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def addTwoNumbers(
self, l1: Optional[ListNode], l2: Optional[ListNode]
) -> Optional[ListNode]:
s1, s2 = [], []
while l1:
s1.append(l1.val)
l1 = l1.next
while l2:
s2.append(l2.val)
l2 = l2.next
dummy = ListNode()
carry = 0
while s1 or s2 or carry:
s = (0 if not s1 else s1.pop()) + (0 if not s2 else s2.pop()) + carry
carry, val = divmod(s, 10)
# node = ListNode(val, dummy.next)
# dummy.next = node
dummy.next = ListNode(val, dummy.next)
return dummy.next
Complexity
The time complexity is O(n). The space complexity is O(n).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.