LeetCode 445. Add Two Numbers II

Question

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

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

https://leetcode.com/problems/add-two-numbers-ii/

• Solution1

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Stack<Integer> stack1 = new Stack<>();
        Stack<Integer> stack2 = new Stack<>();
        while(l1 != null){
            stack1.push(l1.val);
            l1=l1.next;
        }
        while(l2 != null){
            stack2.push(l2.val);
            l2= l2.next;
        }
        ListNode res = null;
        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;
            ListNode cur = new ListNode(sum % 10);
            carry = sum / 10;
            cur.next = res;
            res = cur;
        }
        return res;
    }
}

Complexity:

Time complexity: O(n)

Space complexity: O(n)