[LeetCode] 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 contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

Sol 1: Recursive call the function.

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {

        if (l1 == null && l2 == null) return null;

        int num1 = (l1 == null)? 0 : l1.val;

        int num2 = (l2 == null)? 0 : l2.val;

        int sum = num1 + num2;

        ListNode root = new ListNode(sum % 10);

        int carry = sum / 10;

        if (l1.next != null) {

            l1.next.val += carry;

        } else if (l2.next != null) {

            l2.next.val += carry;

        } else {

            ListNode node = root;

            while (carry > 0) {

                node.next = new ListNode(carry % 10);

                carry = carry / 10;

                node = node.next;

            }

            return root;

        } 

        

        ListNode node1 = (l1 == null)? new ListNode(0) : l1.next;

        ListNode node2 = (l2 == null)? new ListNode(0) : l2.next;

        root.next = addTwoNumbers(node1, node2);

        return root;

}

Sol2: Use while loop to add numbers iteratively.

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {

        if (l1 == null) return l2;

        if (l2 == null) return l1;

        

        ListNode root = new ListNode(0);

        ListNode node = root;

        ListNode p = l1;

        ListNode q = l2;

        int carry = 0;

        while (p != null || q != null) {

            int p_num = (p == null)? 0 : p.val;

            int q_num = (q == null)? 0 : q.val;

            int sum = p_num + q_num + carry;

            carry = sum / 10;

            sum = sum % 10;

            if (p != null) p = p.next;

            if (q != null) q = q.next;

            node.next = new ListNode(sum);

            node = node.next;

        }

        

        while (carry > 0) {

            node.next = new ListNode(carry % 10);

            node = node.next;

            carry = carry / 10;

        }

        

        return root.next;

    }

Remark: Iterative way is much more faster than recursive one in runtime. But the complexity is the same. Recursive way makes more readable / clean code.