0445-AddTwoNumbersII.cs

//-----------------------------------------------------------------------------
// Runtime: 108ms
// Memory Usage: 27.7 MB
// Link: https://leetcode.com/submissions/detail/372014048/
//-----------------------------------------------------------------------------

using System.Collections.Generic;

namespace LeetCode
{
    /**
     * Definition for singly-linked list.
     * public class ListNode {
     *     public int val;
     *     public ListNode next;
     *     public ListNode(int val=0, ListNode next=null) {
     *         this.val = val;
     *         this.next = next;
     *     }
     * }
     */
    public class _0445_AddTwoNumbersII
    {
        public ListNode AddTwoNumbers(ListNode l1, ListNode l2)
        {
            var stack1 = new Stack<int>();
            var stack2 = new Stack<int>();
            while (l1 != null)
            {
                stack1.Push(l1.val);
                l1 = l1.next;
            }
            while (l2 != null)
            {
                stack2.Push(l2.val);
                l2 = l2.next;
            }

            var sum = 0;
            ListNode current = null;
            while (stack1.Count > 0 || stack2.Count > 0 || sum > 0)
            {
                if (stack1.Count > 0)
                    sum += stack1.Pop();
                if (stack2.Count > 0)
                    sum += stack2.Pop();

                var newNode = new ListNode(sum % 10, current);
                current = newNode;
                sum /= 10;
            }

            return current;
        }
    }
}