2.21 Add Two Numbers

Description

You are given two linked lists representing two non-negative numbers. 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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4) Output: 7 -> 0 -> 8

Method

Since numbers is represented in reverse order by linkedlist, we just need to add numbers from the both heads to tails.

So we need a int add for store the add digit if the sum of two digits larget than 10;

and if one list is short, we need do loop the other one's remain nodes;

Time and Space Complexity

o(n + m)

Code

public class Solution {

 public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
       if (l1 == null || l2 == null){
           return l1 == null ? l2 : l1;
       }
       ListNode dummy = new ListNode(0);
       ListNode pre = dummy;
       int add = 0;
       while (l1 != null && l2 != null){
              int v = (l1.val + l2.val + add) % 10;
              ListNode node = new ListNode(v);
              add = (l1.val + l2.val + add) / 10;
              pre.next = node;
              pre = pre.next;
              l1 = l1.next;
              l2 = l2.next;
       }

       while (l1 != null){
              int v = (l1.val + add) % 10;
              ListNode node = new ListNode(v);
              add = (l1.val + add) / 10;
              pre.next = node;
              pre = pre.next;
              l1 = l1.next;
       }

       while (l2 != null){
              int v = (l2.val + add) % 10;
              ListNode node = new ListNode(v);
              add = (l2.val + add) / 10;
              pre.next = node;
              pre = pre.next;
              l2 = l2.next;
       }

       if (add > 0){
           pre.next = new ListNode(add);
       }
       return dummy.next;
}
}