Return to Answer

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.
 final LinkedListNode pointer = new LinkedListNode(0);
 
 int carry = 0;
 LinkedListNode cursor = pointer;
 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) {
 cursor.next = new LinkedListNode(carry);
 }
 
 // don't return the dummy pointer, return the actual result
 return pointer.next;
}

Some side-notes:

• 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.

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.
 final LinkedListNode pointer = new LinkedListNode(0);
 
 int carry = 0;
 LinkedListNode cursor = pointer;
 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) {
 cursor.next = new LinkedListNode(carry);
 }
 
 // don't return the dummy pointer, return the actual result
 return pointer.next;
}

Some side-notes:

• 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.