Adding two numbers represented as linked list

Question

I'm trying to implement a function to add two numbers represented as reverse linked lists. In my opinion the code is correct but on giving input with both linkedlist consisting of just one number [5] . The output is coming out to be [0], but it's supposed to be [0]->[1]

Example First List: 5->6->3 // represents number 365 Second List: 8->4->2 // represents number 248 . Resultant list: 3->1->6 // represents number 613

can someone tell me what I'm doing wrong in the logic?`

/**
 * 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) {
        //find length
        int carry = 0;
        ListNode sum = null;
        int x = 0;
        ListNode top = new ListNode(0);
        while(l1 != null || l2 != null){
            sum = new ListNode((l1.val+l2.val+carry)%10);
//            sum.val = (l1+l2+carry)%10;
            if(x==0){
                top.next = sum;
                x++;
            }
            carry = (l1.val+l2.val)/10;
            l1 = l1.next;
            l2 = l2.next;
            sum = sum.next;
        }
        if(l1 == null && l2 == null){
            //return top;

        }else if(l1 != null && l2 == null){
            while(l1 != null){
                sum = new ListNode((l1.val+carry)%10);
//            sum.val = (l1+carry)%10;
            if(x==0){
                top.next = sum;
                x++;
            }
            carry = (l1.val)/10;
            l1 = l1.next;
            sum = sum.next;
            }
            //return top;
        }else{
            while(l1 != null){
            sum = new ListNode((l2.val+carry)%10);
//            sum.val = (l2+carry)%10;
            if(x==0){
                top.next = sum;
                x++;
            }
            carry = (l2.val)/10;
            l2 = l2.next;
            sum = sum.next;
            }
            //return top;
        }
        if(carry == 1){
            sum = new ListNode(1);

            sum = sum.next;
        }
        return top.next;
    }
}`

First List: 5->6->3 // represents number 365 Second List: 8->4->2 // represents number 248 . Resultant list: 3->1->6 // represents number 613 — SHUBHAM SHARMA
– SHUBHAM SHARMA, Commented May 30, 2015 at 10:03
ok, edit your question based on that, make it simple for someone who wants to help you to understand your problem — Luigi Cortese
– Luigi Cortese, Commented May 30, 2015 at 10:08
please post your full code with insertion and main method so its easy to find the mistake — aakansha
– aakansha, Commented May 30, 2015 at 10:53
There's no full code. I'm solving this problem here --> leetcode.com/problems/add-two-numbers — SHUBHAM SHARMA
– SHUBHAM SHARMA, Commented May 30, 2015 at 11:16

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

Here's a solution (using a "value" method for ListNode):

package so30544570;

public class Solution {
    public static ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        return ListNode.valueOf(l1.value() + l2.value());
    }

    public static void main(String[] args) {
        ListNode l1 = ListNode.valueOf(365);
        ListNode l2 = ListNode.valueOf(248);
        System.out.println("l1 = " + l1);
        System.out.println("l2 = " + l2);
        System.out.println("l1 + l2 = " + addTwoNumbers(l1, l2));
    }
}

And ListNode:

package so30544570;

public class ListNode {
    private int val;
    private ListNode next;

    private ListNode(int x) {
        val = x;
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder().append(val);
        if (next != null) {
            sb.append(" -> ").append(next.toString());
        }
        return sb.toString();
    }

    public int value() {
        int result = val;
        if (next != null) {
            result += 10 * next.value();
        }
        return result;
    }

    public static ListNode valueOf(final int i) {
        ListNode root = new ListNode(i % 10);
        if (i / 10 > 0) {
            root.next = ListNode.valueOf(i / 10);
        }
        return root;
    }
}

Output:

l1 = 5 -> 6 -> 3

l2 = 8 -> 4 -> 2

l1 + l2 = 3 -> 1 -> 6

NB: this solution avoids the sum computation, we just recreate a ListNode for the sum.

edited Jun 20, 2020 at 9:12

CommunityBot

11 silver badge

answered May 30, 2015 at 11:25

user180100

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1 Comment

SHUBHAM SHARMA Over a year ago

Thanks for the help but the question was to be solved on an OJ and the ListNode definition could not have been modified. I have solved it , and figured out the mistake. Thanks again

shanmuga · Accepted Answer · 2015-05-30 10:45:32Z

0

You are never updating top. What you should be doing is

if(carry == 1){
            sum = new ListNode(1);
            if(x==0){
                top.next = sum;   //This line is not executed as x = 1
                x++;
        }

What is x and why is it used.

Note: Name your variable properly for readability.

edited May 30, 2015 at 10:45

answered May 30, 2015 at 10:08

shanmuga

4,5093 gold badges23 silver badges35 bronze badges

4 Comments

SHUBHAM SHARMA Over a year ago

i have to return the top of the linked list which contains the sum. that's why I'm not updating top.

SHUBHAM SHARMA Over a year ago

Yeah that code is anyways not supposed to be executed . X is just used in the upper while loop to update the first node.

shanmuga Over a year ago

That code is one which will add the add [1]. If that s not executed then you will get only [0] (as you are getting now). If it is executed you will get [0]->[1] (as required). I suggest you reconsider your logic (and what you want to achieve using the code)

SHUBHAM SHARMA Over a year ago

if(x==0){top.next = sum; x++;} <== this is not supposed to be executed. The rest i suppose is correct. You;re not being helpful at all

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