// Definition for singly-linked list:
// class ListNode<T> {
//   ListNode(T x) {
//     value = x;
//   }
//   T value;
//   ListNode<T> next;
// }
//
ListNode<Integer> addTwoHugeNumbers(ListNode<Integer> a, ListNode<Integer> b) {
    if (a.value.equals(0) && a.next == null) {
        return b;
    } else if (b.value.equals(0) && b.next == null) {
        return a;
    }

    String aAmount = "";
    while (a != null) {
        aAmount += padValue(a.value);
        a = a.next;
    }

    String bAmount = "";
    while (b != null) {
        bAmount += padValue(b.value);
        b = b.next;
    }

    BigInteger bigAmount = new BigInteger(aAmount).add(new BigInteger(bAmount));

    Stack<Integer> stack = new Stack<Integer>();
    while (!bigAmount.mod(new BigInteger("10000")).equals(bigAmount)) {
        int newValue = bigAmount.mod(new BigInteger("10000")).intValue();
        stack.push(newValue);
        bigAmount = bigAmount.divide(new BigInteger("10000"));
    }

    ListNode<Integer> head = new ListNode<Integer>(0);
    ListNode<Integer> current = head;

    while (!stack.empty()) {
        int newValue = stack.pop();
        ListNode<Integer> newNode = new ListNode<Integer>(newValue);
        current.next = newNode;
        current = newNode;
    }

    head.value = bigAmount.mod(new BigInteger("10000")).intValue();

    return head;
}

String padValue(int value) {
    String result = "";
    if (value > 999) {
        result += String.valueOf(value);
    } else if (value > 99) {
        result += "0" + String.valueOf(value);
    } else if (value > 9) {
        result += "00" + String.valueOf(value);
    } else {
        result += "000" + String.valueOf(value);
    }
    return result;
}