LeetCode OJ 2 Add Two Numbers

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Question

[LeetCode 2] You are given two linked lists representing two non-negative numbers. 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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)

Output: 7 -> 0 -> 8

Submission

Add each node one by one, if the result is greater than 10, carry forward, repeat until both l1, l2 and the carrier are resolved.

Java Submission
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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        return addTwoNumbers(l1, l2, 0);
    }
    
    public ListNode addTwoNumbers(ListNode l1, ListNode l2, int carrier) {
        if (l1 == null && l2 == null) {
            if (carrier > 0) {
                return new ListNode(carrier);
            } else {
                return null;
            }
        }
        
        int l1Val = l1 != null ? l1.val : 0;
        int l2Val = l2 != null ? l2.val : 0;
        int sum = l1Val + l2Val + carrier;
        carrier = sum >= 10 ? 1 : 0;
        sum = sum >= 10 ? sum - 10 : sum;
        
        ListNode result = new ListNode(sum);
        
        if (l1 == null) {
            result.next = addTwoNumbers(null, l2.next, carrier);
        } else if (l2 == null) {
            result.next = addTwoNumbers(l1.next, null, carrier);
        } else {
            result.next = addTwoNumbers(l1.next, l2.next, carrier);
        }
        
        return result;
    }
}