Add two sums

[geekelo]

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.

var addTwoNumbers = function(l1, l2) { // if l1.next_node exists };

[geekelo]

To solve this problem, you need to traverse both linked lists simultaneously, adding the corresponding digits along with any carry from the previous addition. Here's a JavaScript function to achieve this:

var addTwoNumbers = function(l1, l2) {
    let dummyHead = new ListNode(0);
    let current = dummyHead;
    let carry = 0;

    while (l1 !== null || l2 !== null) {
        const x = l1 !== null ? l1.val : 0;
        const y = l2 !== null ? l2.val : 0;
        const sum = x + y + carry;
        carry = Math.floor(sum / 10);

        current.next = new ListNode(sum % 10);
        current = current.next;

        if (l1 !== null) l1 = l1.next;
        if (l2 !== null) l2 = l2.next;
    }

    if (carry > 0) {
        current.next = new ListNode(carry);
    }

    return dummyHead.next;
};

This function takes two linked list nodes, l1 and l2, as input and returns the sum as a new linked list.

Make sure you define the ListNode class before using this function. The ListNode class typically looks like this:

function ListNode(val) {
    this.val = val;
    this.next = null;
}

This code will handle cases where the lengths of the input linked lists are not the same and where there is a carry at the end of the addition.