Understand Add two Numbers Problem

Problem Name: Add two Numbers

Problem Description:

Problem: Add Two Numbers Represented by Linked Lists

Difficulty: Medium
Topics: Linked Lists, Mathematical Computation, Algorithm Design
Objective: Add two non-negative integers represented by linked lists and return the result as a linked list.

Problem Description

You are given two non-negative integers represented by two linked lists, where each node contains a single digit. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

Example:

Input:
l1 = [2, 4, 3], l2 = [5, 6, 4]

Output:
[7, 0, 8]

Explanation:
342 + 465 = 807. The sum is returned as a linked list, where the digits are reversed.

Code Explanation

Step-by-Step Breakdown

The code defines a C++ class called Solution with a member function addTwoNumbers. The function adds two numbers represented by linked lists and returns the result as a new linked list.

1. Initialization:

A dummy head node (dummyHead) is created to simplify list construction.
A pointer (l3) is initialized to point to dummyHead.

2. Addition Loop:

The loop continues as long as either of the linked lists has more digits or there's a carry left.
For each iteration, the digits from l1 and l2 are extracted (or treated as 0 if the list is shorter).
The sum is computed, and the carry is updated.

3. Creating the Result List:

A new node with the current digit is created and appended to the result list.
The pointer l3 is moved forward, and l1 and l2 are also advanced to the next nodes.

4. Final Result:

The result starts from the node following the dummy head.
The dummy head is deleted, and the final result is returned.

Test Cases

Test Case 1: Standard Addition

Input:
l1 = [2, 4, 3], l2 = [5, 6, 4]

Output:
[7, 0, 8]

Explanation:
342 + 465 = 807, which is returned as a linked list: [7, 0, 8].

Test Case 2: Addition with Carry Over

Input:
l1 = [9, 9, 9], l2 = [1, 1, 1]

Output:
[0, 1, 1, 1]

Explanation:
999 + 111 = 1110, which is returned as a linked list: [0, 1, 1, 1].

Test Case 3: Unequal Length Lists

Input:
l1 = [1, 2], l2 = [9, 9, 9]

Output:
[0, 2, 0, 1]

Explanation:
12 + 999 = 1011, which is returned as a linked list: [0, 2, 0, 1].

Test Case 4: One List is Empty

Input:
l1 = [], l2 = [5, 6, 4]

Output:
[5, 6, 4]

Explanation:
If one list is empty, the other list is returned as the result.

Test Case 5: Both Lists are Empty

Input:
l1 = [], l2 = []

Output:
[]

Explanation:
If both lists are empty, the result is an empty list.

Category:
Linked List

Programming Language:
Java

Reference Link:

https://leetcode.com/problems/add-two-numbers/

Online IDE

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Java

Output:

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Tracking code (View only. In case you want to track the code, click this button):

Main Function:

Main Function is not defined.

Helper Function:

INPUT: l1 = [2,4,3], l2 = [5, 6, 4]

OUTPUT: [7,0, 8]

public static ListNode* addTwoNumbers(ListNode* l1, ListNode* l2){

ListNode* dummyHead = new ListNode(0);

ListNode* l3 = dummyHead;

int ex = 0;

while(l1 != nullptr || l2 != nullptr || ex != 0){

int digit1 = (l1 != nullptr)? l1 ->val :0;

int digit2 = (l2 != nullptr)? l2->val:0;

int data = digit1 + digit2 + ex ;

int sum = data % 10;

ex = data /10;

ListNode* temp = new ListNode(sum);

l3 -> next = temp;

l3 = l3 -> next;

l1 = (l1 != nullptr)? l1 ->next : nullptr

l2 = (l2 != nullptr)? l2 -> next: nullptr

}

ListNode* result = dummyHead -> next;

delete dummyHead;

return result;

}//function end

Utility Functions and Global variables:

Utility Function is not required.