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Runtime: 2 ms, faster than 54.87% of Java online submissions for Add Two Numbers.
Memory Usage: 39.5 MB, less than 71.39% of Java online submissions for Add Two Numbers.

public class CommonTest {

    @Test
    public void test(){
        int i1[] = {5,6};
        int i2[] = {5,9,1};
        ListNode test = addTwoNumbers(oneNode(i1), oneNode(i2));
        System.out.println(test);
    }

    public ListNode oneNode(int[] value){
        ListNode node = new ListNode(value[0]);
        if (value.length == 1){
            return node;
        }
        ListNode nextNode = node.next = new ListNode(value[1]);
        for (int i = 2 ; i < value.length ; i ++){
            nextNode = nextNode.next = new ListNode(value[i]);
        }
        return node;
    }

    public class ListNode {
        int val;
        ListNode next;
        ListNode(int x) { val = x; }
        ListNode(int val, ListNode next) { this.val = val; this.next = next; }
    }

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int[] sumInt = new int[2];
        ListNode headNode = new ListNode(sumInt[0]);
        ListNode l1NextNode = l1;
        ListNode l2NextNode = l2;
        ListNode nextNode = headNode;
        while (l1NextNode != null || l2NextNode != null || sumInt[0] == 1){
            if (l1NextNode == null){
                l1NextNode = new ListNode(0);
            }
            if (l2NextNode == null){
                l2NextNode = new ListNode(0);
            }
            sumInt = addTwoInt(l1NextNode.val, l2NextNode.val, sumInt[0]);
            nextNode.val = sumInt[1];

            l1NextNode = l1NextNode.next;
            l2NextNode = l2NextNode.next;
            if (l1NextNode != null || l2NextNode != null  || sumInt[0] == 1){
                nextNode = nextNode.next = new ListNode(0);
            }
        }
        return headNode;
    }

    private int[] addTwoInt(int i1, int i2, int one){
        int[] retInt = new int[2];
        int sum = i1 + i2 + one;
        if (sum >= 10){
            retInt[0] = 1;
            retInt[1] = sum - 10;
        } else {
            retInt[0] = 0;
            retInt[1] = sum;
        }
        return retInt;
    }
}