https://leetcode.com/problems/add-two-numbers/

Keep track of the carry using a variable and simulate digits-by-digits sum starting from the head of list, which contains the least-significant digit.

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/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
const addTwoNumbers = (l1, l2) => {
    let dummy = new ListNode(0);
    let current = dummy;
    let carry = 0;
    let p = l1;
    let q = l2;
    while(p || q || carry) {
        let x = p ? p.val : 0;
        let y = q ? q.val : 0;
        let sum = x + y + carry;
        carry = ~~(sum / 10);
        current.next = new ListNode(sum % 10);
        current = current.next;
        p = p && p.next;
        q = q && q.next;
    }
    return dummy.next;
};