addTwoNumbersLL.java

/**
 * 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) {
        Stack<Integer> s1 = new Stack<>();
        Stack<Integer> s2 = new Stack<>();
        
        while(l1 != null)
        {
            s1.push(l1.val);
            l1 = l1.next;
        }
        
        while(l2 != null)
        {
            s2.push(l2.val);
            l2 = l2.next;
        }
        
        ListNode result = new ListNode(0);
        int sum = 0;
        
        while(!s1.isEmpty() || !s2.isEmpty())
        {
            if(!s1.isEmpty()) sum += s1.pop();
            if(!s2.isEmpty()) sum += s2.pop();
            result.val = sum%10;
            ListNode cur = new ListNode(sum/10);
            cur.next = result;
            result = cur;
            sum /= 10;            
        }
        
        return result.val == 0? result.next:result;        
    }
}