ListNode.java

package com.leetcode;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

import java.util.ArrayList;
import java.util.List;

public class ListNode {
 int val;
 ListNode next;

 ListNode() {
 }

 ListNode(final int val) {
 this.val = val;
 }

 ListNode(final int val, final ListNode next) {
 this.val = val;
 this.next = next;
 }

 ListNode(final int[] nums) {
 ListNode dummy_head = new ListNode();
 ListNode curr_node = dummy_head;
 for (final var num: nums) {
 curr_node.next = new ListNode(num);
 curr_node = curr_node.next;
 }
 this.val = dummy_head.next.val;
 this.next = dummy_head.next.next;
 }

 int [] to_array() {
 List<Integer> numbers = new ArrayList<>();
 var curr_node = this;
 while (curr_node != null) {
 numbers.add(curr_node.val);
 curr_node = curr_node.next;
 }
 return numbers.stream().mapToInt(i -> i).toArray();
 }
}

class TestListNode {
 @Test
 void test_to_array() {
 final var nums = new int[] { 1,2,3};
 final var node = new ListNode(nums);
 Assertions.assertArrayEquals(node.to_array(), nums);
 }

 // https://leetcode.com/problems/add-two-numbers
 public static ListNode add_two_numbers(ListNode l1, ListNode l2) {
 ListNode dummyHead = new ListNode();
 ListNode current_node = dummyHead;
 int sumOrCarry = 0;
 while (l1 != null || l2 != null || sumOrCarry != 0) {
 if (l1 != null) {
 sumOrCarry += l1.val;
 l1 = l1.next;
 }
 if (l2 != null) {
 sumOrCarry += l2.val;
 l2 = l2.next;
 }
 current_node.next = new ListNode(sumOrCarry % 10);
 current_node = current_node.next;
 sumOrCarry /= 10;
 }
 return dummyHead.next;
 }

 @Test
 void test_add_two_numbers() {
 final ListNode ln1 = new ListNode(new int[]{5});
 final ListNode ln2 = new ListNode(new int[]{5});
 Assertions.assertArrayEquals(add_two_numbers(ln1, ln2).to_array(), new int[]{0, 1});
 }
}