Problem

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.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

Solution

Following is my solution, as long as you can deal with the carry well, you can solve the problem easily.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode p1 = l1, p2 = l2;
        int carry = 0;
        ListNode result = null;
        ListNode p = result;
        for(; p1 != null || p2 != null; ) {
            int val1 = (p1 != null) ? p1.val : 0;
            int val2 = (p2 != null) ? p2.val : 0;
            
            int sum = val1 + val2 + carry;
            carry = sum / 10;
            if(result == null) {
                result = new ListNode(sum % 10);
                p = result;
            } else {
                p.next = new ListNode(sum % 10);
                p = p.next;
            }
            
            p1 = p1 != null ? p1.next : p1;
            p2 = p2 != null ? p2.next : p2;
            
        }
        
        if(carry != 0) {
            p.next = new ListNode(carry);
        }
        
        return result;
        
    }
}

Solution using a dummy head

As you can see in my solution, the leading node of the linked list is a special case and requires an extra case. To make it easier, a dummy head can be used as in the solution provided by Leetcode.

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    ListNode dummyHead = new ListNode(0);
    ListNode p = l1, q = l2, curr = dummyHead;
    int carry = 0;
    while (p != null || q != null) {
        int x = (p != null) ? p.val : 0;
        int y = (q != null) ? q.val : 0;
        int sum = carry + x + y;
        carry = sum / 10;
        curr.next = new ListNode(sum % 10);
        curr = curr.next;
        if (p != null) p = p.next;
        if (q != null) q = q.next;
    }
    if (carry > 0) {
        curr.next = new ListNode(carry);
    }
    return dummyHead.next;
}