LL 15 - Add Two Numbers

Problem

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.

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.

Solution

We take corresponding nodes from the two lists, calculate the resultant sum, and place it in a new node, which is then added to the answer head linked list. If any carry is generated, we account for it in the next node addition. Initially, we use a dummy node for the answer linked list as the head to reduce condition checks and improve code readability.

Time - O(n)

Space - O(n)

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {

        ListNode temp1 = l1;
        ListNode temp2 = l2;

        int carry = 0;

        ListNode head = new ListNode(-1);
        ListNode curr = head;

        while(temp1!=null || temp2!=null){
            int sum = carry;

            if(temp1!=null) sum += temp1.val;
            if(temp2!=null) sum += temp2.val;

            carry = sum/10;

            ListNode newNode = new ListNode(sum%10);
            curr.next =newNode;
            curr = newNode;

            if(temp1!=null) temp1 = temp1.next;
            if(temp2!=null) temp2 = temp2.next;
        }

        if(carry==1){
            ListNode newNode = new ListNode(1);
            curr.next = newNode;
        }

        return head.next;
    }
}