Add Two Numbers: the idea to remember.

The core insight

Advance both lists together, emit sum modulo ten, and carry the quotient to the next position.

Why it works

At each position, the two digits plus incoming carry are precisely the contribution to that decimal place.

The mistake to avoid

Continue while either list or the carry remains; the lists may have different lengths.

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

// Java reference adapted from neetcode-gh/leetcode (MIT).
// Advance both lists together, emit sum modulo ten, and carry the quotient to the next position.
public ListNode addTwoNumbers(ListNode first, ListNode second) {
    int q = 0;
    int r = 0;
    int sum = 0;
    ListNode head = null;
    ListNode temp = null;
    while (first != null || second != null || q>0) {
        sum =
            q +
            (
                ((first != null) ? first.val : 0) +
                ((second != null) ? second.val : 0)
            );
        r = sum % 10;
        q = sum / 10;
        ListNode newNode = new ListNode(r);
        if (head == null) {
            head = newNode;
        } else {
            temp = head;
            while (temp.next != null) {
                temp = temp.next;
            }
            temp.next = newNode;
            newNode.next = null;
        }
        if (first != null) {
            first = first.next;
        }
        if (second != null) {
            second = second.next;
        }
    }
    return head;
}

Java reference adapted from neetcode-gh/leetcode · MIT license

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