LeetCode 2: Add Two Numbers

Tips: Tried to use a private method to simplize the code. But found that object is passed by reference. So even if you have ListNode as l, and l = l.next, it is reference change. It will not impac the object in caller function.

 

/**
 * 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) {
        if (l1 == null) {
            return l2;
        }
        
        if (l2 == null) {
            return l1;
        }
        ListNode result = new ListNode(0);
        ListNode tail = result;
        int c = 0;
        while (l1 != null || l2 != null) {
            int n1 = 0;
            int n2 = 0;
            if (l1 != null) {
                n1 = l1.val;
                l1 = l1.next;
            }
            
            if (l2 != null) {
                n2 = l2.val;
                l2 = l2.next;
            }
            tail.next = new ListNode((n1 + n2 + c)%10);
            tail = tail.next;
            c = (n1 + n2 + c) / 10;
        }
        
        if (c > 0) {
            tail.next = new ListNode(c);
        }
        
        return result.next;
    }
}