2. Add Two Numbers.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) {
        if(l1 == null || l2 == null){
            return l1 == null ? l2 : l1;
        }
        
        ListNode result = new ListNode();
        ListNode ans = result;
        int carry=0;
        while(l1 != null && l2 != null){
            
            int sum = carry + l1.val + l2.val;
            int digit = sum%10;
            carry = sum/10;
            
            l1 = l1.next;
            l2 = l2.next;
            
           result.next = new ListNode(digit);   
            result = result.next;
        }
        
        while(l1 != null){
            int sum = carry + l1.val;
            int digit = sum%10;
            carry = sum/10;
            result.next = new ListNode(digit);
            l1 = l1.next;
            result = result.next;
        } 
        
        while(l2 != null){
            int sum = carry + l2.val;
            int digit = sum%10;
            carry = sum/10;
            result.next = new ListNode(digit);
            l2 = l2.next;
            result = result.next;
        } 
        
        if(carry != 0){
            result.next = new ListNode(carry);
        }
        
        return ans.next;
    }
}