[LeetCode] Problem 445 - Add Two Numbers II

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You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Follow up

What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

Example

Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)

Output: 7 -> 8 -> 0 -> 7

Code

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public class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
}
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public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    Stack<Integer> stack1 = new Stack<>();
    Stack<Integer> stack2 = new Stack<>();
    ListNode dummy = new ListNode(0);

    while (l1 != null) {
        stack1.push(l1.val);
        l1 = l1.next;
    }

    while (l2 != null) {
        stack2.push(l2.val);
        l2 = l2.next;
    }

    int carry = 0;

    while (!stack1.isEmpty() || !stack2.isEmpty() || carry != 0) {
        int val1 = stack1.isEmpty() ? 0 : stack1.pop();
        int val2 = stack2.isEmpty() ? 0 : stack2.pop();
        int sum = val1 + val2 + carry;
        carry = sum / 10;

        ListNode node = new ListNode(sum % 10);
        node.next = dummy.next;
        dummy.next = node;
    }

    return dummy.next;
}