Leetcode #2: Add Two Numbers
In this guide, we solve Leetcode #2 Add Two Numbers 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 digits are stored in reverse order, and each of their nodes contains a single digit.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Recursion, Linked List, Math
Intuition
There is a mathematical invariant or formula that directly leads to the result.
Using math avoids unnecessary loops and reduces complexity.
Approach
Derive the formula or update rule, then compute the answer directly.
Handle edge cases like overflow or zero carefully.
Steps:
- Identify the math relationship.
- Compute the result with a loop or formula.
- Handle edge cases.
Example
Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.
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]:
dummy = ListNode()
carry, curr = 0, dummy
while l1 or l2 or carry:
s = (l1.val if l1 else 0) + (l2.val if l2 else 0) + carry
carry, val = divmod(s, 10)
curr.next = ListNode(val)
curr = curr.next
l1 = l1.next if l1 else None
l2 = l2.next if l2 else None
return dummy.next
Complexity
The time complexity is , where and are the lengths of the two linked lists. The space complexity is .
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.