add_two_numbers_leetcode2.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy_head = new ListNode(0);
        ListNode l3 = dummy_head;
        int carry = 0;
        
        while(l1 != null || l2!=null){
            int l1_val = (l1!=null) ? l1.val : 0;
            int l2_val = (l2!=null) ? l2.val : 0;
            
            int current_sum = l1_val + l2_val + carry;
            carry = current_sum / 10;
            int last_digit = current_sum % 10;
            
            ListNode new_node = new ListNode(last_digit);
            l3.next = new_node;
            
            if(l1 != null) l1=l1.next;
            if(l2 != null) l2=l2.next;
            l3 = l3.next;
        }
        if(carry > 0){
            ListNode new_node = new ListNode(carry);
            l3.next = new_node;
            l3 = l3.next;
        }
        return dummy_head.next;
    }
}