I am practicing for interviews and I tried solving the problem on my own and got a solution. I was wondering how can I improve the performance and memory on this program?

The problem performs addition on linked list nodes. The nodes are 1s->10s->100s->.... The goal of this program is to perform addition on two of these linked lists. The output should be a linked list of the same format.

Example:

  1 -> 4 -> 3
+ 1 -> 5 -> 9 -> 2


2 -> 9 -> 2 -> 3


Here is my code:

public class prob2_5 {
/*
* You have two numbers represented by a linked list, where each node contains a single digit.
* The digits are stored in reverse order, such that the 1's digit is at the head of the list.
* Write a function that adds the two numbers and returns the sum as a linked list
*
*/

int l1sum = 0;
int l2sum = 0;
int multiplier = 1;

while(l1 != null) {
l1sum = l1sum + l1.value*multiplier;
multiplier = multiplier*10;
l1 = l1.next;
}
multiplier = 1;
while(l2 != null) {
l2sum = l2sum + l2.value*multiplier;
multiplier = multiplier*10;
l2 = l2.next;
}

int total = l2sum + l1sum;

char[] totalnum = (total + "").toCharArray();
int len = totalnum.length;
System.out.print(totalnum);

for (int i = len - 2; i >= 0; i--) {
newNode = new LinkedListNode(totalnum[i] - '0');
prevNode.next = newNode;
prevNode = newNode;
}
return tail;
}
}


Here is my linked list class:

public class LinkedListNode {
int value;
this.next = null;
this.value = value;
}
}

• What is your LinkedListNode implementation? Could you provide a runnable example? I didn't really understand your explanation completely. Commented Feb 6, 2014 at 18:25
• Sorry Ill add more to my post Commented Feb 6, 2014 at 18:34
• OK, now I understand. 143 really represents the number 341, and 1592 is 2951. 2951 + 341 = 3292, which is 2923 backwards. Commented Feb 6, 2014 at 18:38
• yes exactly! =D Commented Feb 6, 2014 at 18:48

I believe you are missing the point of the exercise - instead of 'decoding' the input numbers, and then 're-encoding' the output number - you are supposed to iteratively get to the answer by going over the nodes of both lists:

public static LinkedListNode addLists(LinkedListNode l1, LinkedListNode l2) {
}

// stop conditions
if (l1 == null && l2 == null && carryOver == 0) {
return null;
}
if (l1 == null) {
}
if (l2 == null) {
}

// iteration
int addedValue = l1.value + l2.value + carryOver;
carryOver = 0;

carryOver = 1;
}

// recursion

return l3;
}

• Instead of l1 = new LinkedListNode(0), I'd suggest creating a null object: private static final NULL_NODE = new LinkedListNode(0); Commented Feb 6, 2014 at 19:58

Uri has a good answer, this answer is just to add a second spin on the problem....

Recursion is a good tool to have, but it is not necessarily the best tool for all occasions. In this instance, it's a toss-up.... but the iterative solution to this problem is perhaps a simpler thing to read..... and should be considered. Additionally, in many cases an iterative solution will outperform a recursive solution. In this case, again, it is a toss-up because the lists will be so short.... but, if the list was long, the recursive approach will fail with a stack-overflow... the iterative approach will keep trucking though.

public static final LinkedListNode add(LinkedListNode a, LinkedListNode b) {
// this pointer points to the result, it is not the actual result.

int carry = 0;
while (a != null || b != null) {
int digitsum = carry;
if (a != null) {
digitsum += a.value;
a = a.next;
}
if (b != null) {
digitsum += b.value;
b = b.next;
}
cursor.next = new LinkedListNode(digitsum % 10);
carry = digitsum / 10;
cursor = cursor.next;
}
if (carry != 0) {

• the variables l1 and l2 are not great names....
• The LinkedListNode should probably have a final value, and there should be getters for the value and next, and a setter for the next.