2. Add Two Numbers

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]

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(-1);
        ListNode cur = dummy;

        int carry = 0;

        while(l1 != null && l2 != null){
            int curSum = l1.val + l2.val + carry;

            carry = curSum / 10;
            curSum = curSum % 10;

            cur.next = new ListNode(curSum);
            cur = cur.next;
            l1 = l1.next;
            l2 = l2.next;
        }

        while(l1 != null){
            int curSum = l1.val + carry;
            carry = curSum / 10;
            curSum = curSum % 10;
            cur.next = new ListNode(curSum);
            cur = cur.next;
            l1 = l1.next;

        }

        while(l2 != null){
            int curSum = l2.val + carry;
            carry = curSum / 10;
            curSum = curSum % 10;
            cur.next = new ListNode(curSum);
            cur = cur.next;
            l2 = l2.next;
        }

        if (carry != 0){
            cur.next = new ListNode(carry);
            cur = cur.next;
        }

        return dummy.next;

    }
}
// TC: O(n)
// SC: O(1)

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode curr = dummy;

        int carry = 0;

        while (l1 != null || l2 != null || carry ==1){
            int sum = 0;
            if (l1 != null){
                sum = sum +l1.val;
                l1 = l1.next;
            }

            if (l2 != null){
                sum = sum + l2.val;
                l2 = l2.next;
            }
            sum = sum + carry;
            carry = sum/10;
            ListNode node = new ListNode(sum % 10);
            curr.next = node;
            curr = curr.next;
        }
        return dummy.next;


    }
}

// TC(O(n))
// SC(O(1))