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LeetCode Add Two Numbers

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. Add the two numbers and return the sum as a linked list.

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

Problem statement taken from: https://leetcode.com/problems/add-two-numbers

Example 1:

Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.

Example 2:

Input: l1 = [0], l2 = [0]
Output: [0]

Example 3:

Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
Output: [8,9,9,9,0,0,0,1]

Constraints:

  - The number of nodes in each linked list is in the range [1, 100].
  - 0 <= Node.val <= 9
  - It is guaranteed that the list represents a number that does not have leading zeros.

Explanation

The numbers are represented in reverse order in LinkedList and hence we do not have to worry about reversing the list. The head of the LinkedList represent the least-significant digit of the numbers.

Just like we add two numbers in Mathematics on a piece of paper, we begin summing the least-significant digits. As per the given constraint, each digit in the node is in the range 0..9 so the sum may overflow.

For e.g., 4 + 9 = 13. In this case, we set the current node digit to 3 and carry 1 to the next iteration. The maximum sum can be 9 + 9 = 18. So carry will be either 1 or 0.

Algorithm
- Initialize sum to 0.
- Initialize a current node which acts as a iterator and set the current sum as node val.
- Initialize pointer result which points to current node.
- Loop while(l1 != nil || l2 != nil)
  - if ( l1 != nil )
     - Add l1-> val to sum as sum += l1->val
     - Move l1 to point next node l1 = l1->next
  - if ( l2 != nil )
     - Add l2-> val to sum as sum += l2->val
     - Move l2 to point next node l2 = l2->next
  - Initialize new ListNode with last digit of sum new ListNode( sum % 10 )
    - Assign current node next to the new ListNode created above
      - current->next = new ListNode( sum % 10 )
  - Set current to current->next
  - Set sum = sum / 10. sum / 10 will be 0 for sum < 10 and 1 for sum >= 10
- Check if sum > 9, if so append a new node with digit 1 to current node.
- Return result->next since result is still pointing to current's first position.
C++ solution
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        int sum = 0;
        ListNode result(0);
        ListNode *current = &result;

        while(l1 || l2){
            if(l1){
                sum += l1->val;
                l1 = l1->next;
            }

            if(l2){
                sum += l2->val;
                l2 = l2->next;
            }

            current->next = new ListNode(sum % 10);
            current = current->next;
            sum = sum / 10;
        }

        if(sum > 0){
            current->next = new ListNode(sum / 10);
        }

        return result.next;
    }
};
Golang solution
func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
    sum := 0
    current := new(ListNode)
    result := current

    for l1 != nil || l2 != nil {
        if l1 != nil {
            sum = sum + l1.Val
            l1 = l1.Next
        }

        if l2 != nil {
            sum = sum + l2.Val
            l2 = l2.Next
        }

        current.Next = &ListNode{sum % 10, nil}
        current = current.Next
        sum = sum / 10
    }

    if sum > 0 {
        current.Next = &ListNode{sum, nil}
    }

    return result.Next
}
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