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

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var addTwoNumbers = function(l1, l2) {
    const dummy = new ListNode();
    let cur = dummy;
    let carryOver = 0;

    while (l1 || l2) {
        let val = 0;
        if (!l1) {
            val = l2.val + carryOver;
            l2 = l2.next;
        } else if (!l2) {
            val = l1.val + carryOver;
            l1 = l1.next;
        } else {
            val = l1.val + l2.val + carryOver;
            l1 = l1.next;
            l2 = l2.next;
        }
        cur.next = new ListNode(val % 10);
        cur = cur.next;
        carryOver = Math.floor(val / 10);
    }

    if (carryOver) { // for last possible carry over
        cur.next = new ListNode(carryOver);
    }
    return dummy.next;
};

We keep track of the carry using carryOver and simulate digits-by-digits sum starting from the head of list.

Problem​

Solution​

Problem

Solution