Perform Addition

Consider the length of two linked list.

while(ll1.Count != ll2.Count)
{
    if (ll1.Count < ll2.Count)
    {
        ll1.AddFirst(0);
    }
    else
        ll2.AddFirst(0);
}

Main class.

LinkedList<int> ll1 = new LinkedList<int>();
ll1.AddLast(7);
ll1.AddLast(5);
ll1.AddLast(9);
ll1.AddLast(4);
ll1.AddLast(6);
LinkedList<int> ll2 = new LinkedList<int>();
ll2.AddLast(8);
ll2.AddLast(4);
LinkedList<int> ll3 = new LinkedList<int>();

Last, for loop method.

int temp = 0, carry = 0;

Console.WriteLine(ll1.ElementAtOrDefault(4));
for (int i = ll2.Count-1; i>=0; i--)
{
    temp = ll1.Last() + ll2.Last();
    ll3.AddFirst((temp + carry) % 10);
    carry = (temp + carry) / 10;
    ll1.RemoveLast();
    ll2.RemoveLast();
}